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ESTELON FORZABEAUTYTALENT
D’AGOSTINO’SNEWPROGRESSION
MBL’S SMALLER
RADIALSTRAHLER
PSB: HIGH END,
HIGH VALUE❱ NOVEMBER 2021
$8.99 US$10.99 Canada

DS AUDIO:
ANALOG
SEES THE
LIGHT

EQ UIPMENT R EPORTJASON VICTOR SERINUS

D’Agostino Progression M550
MONOBLOCK POWER AMPLIFIER

W

e audiophiles
so frequently
get caught up
in the pursuitof perfection that some haveattempted to rebrand high-end
audio as “perfectionist audio.”
But is it even possible for asingle piece of audio gear, letalone an entire audio system,
to attain perfection when
there’s no common agreement
as to what “perfection” means?
It’s easier to cue up a Nirvana
track than to find the way toaudio nirvana.
Nonetheless, the journey toward sonic perfection
When an amp
continues to grip many of us
like a Siren who lures sailors to
can deliver
their death with the sweetness
all that fromof her song. For some who
listen closely and steer wisely,a simplehowever, that death can be
performance
more akin to La petite mort thanwith voice andto a dead end.
I like to set sail with people
piano, it’s awhose sonic values, as manigreat amp.
fest in the equipment they
design, mirror my own. In
the field of amplification, few American designers are more
known for the pursuit of a certain vision of perfection thanDan D’Agostino. Over a 40+-year history in audio, which
includes founding and serving as CEO and chief engineerof Krell Industries (1980–2009) before moving on to foundDan D’Agostino Master Audio Systems (aka D’Agostino) in
mid-2010, Dan1 has envisioned, designed, and shepherded

the development of a host of solid state amplifiers and otherproducts that have established a benchmark for melding
power and speed with sonic beauty, tonal accuracy, and bass
response.
“I want to make something that sounds musical—that satisfies and brings me closer to the musical event that’s being
reproduced,” Dan said at the start of a phone chat that also
included Senior Engineer Burhan Coskun, who did crucial
work on D’Agostino’s latest designs. “I’ve always strived forthis goal. The closer I get, the more engrossed I get in making it better.
“Where I am is way farther than I’ve ever been. With
Krell, I never achieved sonically what I’ve achieved withDan D’Agostino. Dan D’Agostino is much, much, much
more musical and way closer to my sonic goal.
1 To avoid confusion between Dan D’Agostino the man and Dan D’Agostino thecompany, I’m identifying the former by his first name.

SPECIFICATIONS
Description Solid stateclass-AB mono power amplifier. Inputs: XLR. Outputs:
Custom binding posts. Input
impedance: 100k ohms.
Output impedance: 0.1 ohms.
Power: 550W into 8 ohms,
1100W into 4 ohm, 2200W
into 2 ohms (all 27.4dBW).
Frequency response: 1Hz–

stereophile.com n November 2021

80kHz, –1dB; 20Hz–20kHz,±0.01dB. S/N ratio: 105dB
unweighted, 75dB A-weighted. Distortion: 0.15% @ 1kHz
at 550W into 8 ohms. Powerdraw at idle: 80W.
Dimensions 17.875"(454mm) W × 9.0" (230mm) H × 23" (584mm) D.
Weight: 115lb (52.2kg).

Finish Silver or Black; custom
finishes upon request.
Serial numbers of units
reviewed PM0506P,
PM0507P. Manufactured in
the United States.
Price $44,950/pair. Approximate number of US dealers:
27. Warranty: Three years
from date of retail purchaseor four years from date of
manufacture for original
purchaser of a new productfrom an authorized dealer.
Manufacturer Dan
D’Agostino Master Audio
Systems, LLC. 5855 E SurreyDr., Cave Creek, AZ 85337.
(Tel): (480) 575-3069.
Web: dandagostino.com.

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D’AGOSTINO PROGRESSION M550

“Our current designs have absolutely nothing in common withwhat I did at Krell. When I left thatcompany, I wanted to do something
in the completely opposite direction. I’m glad in a way that I
was asked to leave, because[the move] provided me withthe platform that I could use
to do the designs I do today. I
think that at Krell we got involvedin a numbers game: We can do thisbetter because it measures better, andit does this better. It was more about
technology than listening. But at
D’Agostino, we listen a lot. Listeningis the most important thing, not how
it measures.”
After Dan founded Dan
D’Agostino Master Audio Systems,
the Momentum monoblocks 2 led the way in the first halfof 2011. D’Agostino subsequently developed three tiers of
products: Relentless, Momentum, and Progression. The
monoblock amplifier line, topped by the 570lb Relentlessmonoblocks ($295,000/pair), until recently extended down
through the 95lb Momentum M400 ($65,000/pair) to the
125lb Progressions ($38,000/pair)—my reference amplifiers

since I reviewed them in October
2017. The Progressions are among
the best sounding amps intheir price range that haveever graced my system.
Now the company
has replaced the original
Progression monos withthe fully balanced, 115lb Progression M550 monoblocks($44,950/pair). The class-AB
M550, which requires a 20A
power cable, has a new input-stagetransistor structure that is claimed
to handle a lot more current andprovide six times more power,
greater thermal stability during
power delivery, “perfect” gain
matching between the improved
high-signal N and P–channel transistors, and an improved high-frequency response that melds
lower saturation voltage with higher gain. There’s also an
upgraded output stage that includes 48 power transistors anda 2000VA power supply transformer with nearly 100,000
microfarads of power supply storage capacitance. Together
2 Later called the Momentum M300s.

MEASUREMENTSF

or logistical reasons, I measured a different sample of the
Dan D’Agostino Master Audio
Systems Progression M550
monoblock (serial number PM006P)
with my Audio Precision SYS2722 system (see the January 2008 “As We See
It 1 ”). I repeated some of the measurements with the magazine’s Audio Precision APx500 analyzer. Usefully, the box
included a set of tests performed with
the APx500.
Before I performed the testing, I preconditioned the amplifier by running it
at one-eighth its specified power into
8 ohms for 30 minutes, as recommended by the Consumer Electronics

Association. After the preconditioning,
the M550’s top panel was surprisingly
cool, at 96.8°F (36°C). The temperature of the massive heatsinks on theamplifier’s sides was 101.8°F (38.8°C)
and didn’t increase significantly withthe amplifier idling.
The voltage gain into 8 ohms forthe balanced input was 29.7dB, andthe input inverted absolute polarity, the opposite of what I had foundwith earlier D’Agostino amplifiers.2
The input impedance at low andmiddle frequencies was very slightly
lower than the specified 100k ohms,
at 95k ohms, dropping inconsequentially to 50k ohms at 20kHz. The

output impedance was higher than
the specified 0.1 ohm, at 0.37 ohm at
20Hz and 1kHz, rising slightly to 0.4
ohm at 20kHz. The response with
our standard simulated loudspeaker3
therefore varied by ±0.26dB (fig.1, graytrace). The amplifier offered a wide
small-signal bandwidth, the output into
1 See stereophile.com/content/measurementsmaps-precision.
2 In measuring the Momentum HD, I found it
inverted polarity, but in a Manufacturers’ Comment,
Dan D’Agostino wrote that this was due to an error
in the preproduction code that was fixed in all
production units. See stereophile.com/content/
dan-dagostino-master-audio-systems-momentumhd-line-preamplifier-measurements.
3 See stereophile.com/content/real-life-measurements-page-2.

d
B
r

d
B
r

A

A

Hz

Fig.1 Dan D’Agostino Progression M550, frequency
response at 2.83V into: simulated loudspeaker load(gray), 8 ohms (blue), 4 ohms (magenta), 2 ohms(red) (0.5dB/vertical div.).

stereophile.com n November 2021

Hz

Fig.2 Dan D’Agostino Progression M550, smallsignal, 10kHz squarewave into 8 ohms.

Fig.3 Dan D’Agostino Progression M550, spectrumof 1kHz sinewave, DC–1kHz, at 1W (blue) and at
100W (red) into 8 ohms (linear frequency scale).

54

D’AGOSTINO PROGRESSION M550

with a new and far more efficient heatsink design borrowed
from the Relentless, these changes enable the Progression
M550’s transistors to handle a lot more current flow and run
hotter than previous devices used by D’Agostino. “We’repushing a lot harder on all the circuits, but our safe operating area has actually gotten bigger,” Dan said.
According to Coskun, technological advances in the
Progression M550s have made possible what amounts to a“new amplifier” with a higher bias setting. The M550s canremain in class-A for up to the first 100W of their output.
When used in a moderately sized room—mine is 16' × 20'
with ceilings that flatten out at 9'—the Progression M550s
can deliver 550Wpc into 8 ohms, 1100Wpc into 4 ohms,
and a whopping 2200Wpc into 2 ohms. Operating in classA avoids the switching distortion that is an inevitable resultof operating in class-B at higher power levels. Yet, because
the M550 is a class-AB design, it consumes only 80W at
idle.3 They’re not the Relentless monsters, which start at
1500Wpc into 8 ohms and keep doubling down from there,
but they are still a significant advance for products presented
by a man who has been pursuing what he refers to as “audio
pleasure” for his entire career.
“In all my products, I like to put enough class-A in[the output stage] so that at low, low levels, the amplifier
maintains its sweet presentation and sounds more musical,”Dan explained. “I’m certainly not advocating pure class-A
operation, which is very inefficient. The techniques I’ve de-veloped do not necessitate the use of pure class-A circuitry,
because I get a very musical sound without being in class-A
all the time and using a tremendous amount of energy andhuge heatsinks. Our designs give you the best of high-powered delivery when needed, as well as the sonic accuracy
that class-A bias delivers.”
Dan told me that if he’d listened to the new Progression M550s when he designed the original Momentum
monoblocks 10 or 11 years ago, he would have declared
the Progression M550s better. Nonetheless, in the current
lineup, price reflects sound quality. “All three amps are very
musical,” he insisted, “but they’re all absolutely different
designs that we borrow from [as we upgrade]. If part of acircuit works really well, we will incorporate it into an existing or future design to see if we can improve it.
“The Momentum 400 is definitely another step toward
ultimate musicality—it has a mammoth soundstage, sits on
a warm palette, and has more harmonic texture to it. We’renot upgrading the Relentless; we’re kind of in stasis mode
because we don’t know how to improve it. The Relentless
is a completely different-sounding amplifier with a different
presentation. It’s big and musical with a really solid platform,
but it seems as ultradelicate as listening to a small 10W amplifier on a small speaker. The unfortunate thing is that theRelentless is very heavy. It takes a crew to get them in your
3 For additional information, please see dandagostino.com/products/progressionmono-amplifier.

measurements, continued

8 ohms (blue trace) not reaching –3dB
until 140kHz. The Progression M550’s
reproduction of a 10kHz squarewave(fig.2) accordingly featured very shortrisetimes with, commendably, no apparent overshoot or ringing. (This issomething that has been a consistentfeature with amplifiers designed by
Dan D’Agostino.)
Measured with both phases of thebalanced input shorted to ground, theM550’s wideband, unweighted signal/
noise ratio was 55.3dB, ref. 1W into 8
ohms. This ratio improved to 80.5dB
when the measurement bandwidth was
restricted to 22Hz–22kHz, and to 89dB
when the reading was A-weighted. The
blue trace in fig.3, taken at 1W into 8

ohms, reveals that the primary source ofnoise is magnetic interference at 60Hz
and its odd-order harmonics, which I
assume stem from the massive toroidaltransformer. However, at high powers(red trace), the spuriae at 120Hz and its
harmonics increase in level.
When I examined how the percentage of THD+N varied with output
power, with clipping defined as when
the THD+noise in the amplifier’s
output reached 1%, the M550 clipped
at 555W into 8 ohms (27.44dB, fig.4),
which is marginally higher than thespecified 550W (27.4dBW). The clipping power into 4 ohms (fig.5) was
840W (26.23dBW); though this is
1.17dB lower than the specified 1.1kW,

I don’t hold the wall voltage constantduring these tests, feeling that this is
more representative of an amplifier’s
behavior in a typical system. The wall
voltage was 118.7V with the amplifier
idling but had dropped to 115.1V withthe amplifier clipping into 4 ohms.
Note from these two graphs that the
distortion percentage at low powers ishigher than it is at high powers. Thisbehavior was consistent with both Audio Precision analyzers and is almost
identical to what I found with the Dan
D’Agostino Progression Mono amplifierthat JVS reviewed in October 2017.4

%

%

%

W

Fig.4 Dan D’Agostino Progression M550, distortion(%) vs 1kHz continuous output power into 8 ohms.

stereophile.com n November 2021

W

Fig.5 Dan D’Agostino Progression M550, distortion(%) vs 1kHz continuous output power into 4 ohms.

4 See stereophile.com/content/dan-dagostinoprogression-mono-monoblock-power-amplifiermeasurements.

Hz

Fig.6 Dan D’Agostino Progression M550, THD+N(%) vs frequency at 28.3V into: 8 ohms (blue), 4
ohms (magenta), 2 ohms (red).

55

D’AGOSTINO PROGRESSION M550

house. And they’re very expensive as amplifiers go.
“We keep looking at the M400 to see how we can
improve it, because it’s my baby; it’s my Phoenix of an amplifier that brought me back as a company. But I don’t thinkyou’ll see anything migrating from the new Progression
M550 into the Momentum, because we borrowed from theMomentum to develop the Progression M550, and we borrowed from the Relentless to make the M400. I guess thesame thing happens with automakers.”
Several times during our chat, Dan was effusive in his
praise for Coskun, his chief engineer. “Now that I have
Burhan, who is so astute with circuits, I conceptualize what
I want to do,” he said. “Then I draw up a circuit and ask him
what he thinks. We decide what looks good and where wemight try to change it before we make one. After we get it
where we like it, we simulate it to make sure it does what
it’s supposed to do. Then we build the board, and then welisten to it. That’s exactly what we did with our new Relentless preamplifier, which will perhaps ship in September. It
makes an extraordinary difference to have someone who
understands circuitry working with you. I had 11 engineers
at Krell, but none was as keen on circuits as Burhan.”What about the conundrum of D’Agostino products thatmany, including myself, believe sound great yet which do
not yield stellar measurements on Technical Editor John
Atkinson’s equipment? While the D’Agostino website
claims that the Progression’s “distortion, signal/noise ratio,channel separation, and bandwidth measurements have all
improved,” that doesn’t guarantee that JA1 will find theM550’s measurements even close to superb.
Dan responded, “What I always say is that our equipment measures adequately. The measurements are good andrespectable. But we’re not looking for measurements.
“I use the same Audio Precision gear that John does—
mine may be bigger—but I certainly don’t use it for listening. Believe me, if I want to make low distortion, there’s awhole lot of things I can do. However, in my experienceof 40 some years, low distortion never makes it soundbetter. Ever. Obviously, if you’re way high in distortion, it
won’t sound that good to your ear. But I’m not sure thathigh distortion measurements have anything to do withanything other than bad sound. The M550s will measure
better than the original Progression monoblocks, but just by
happenstance. As Burhan will tell you, we use test equipment as a reference, but it’s to make sure that the circuits wedesign work within their envelope completely the way weintended them to.”
What the good man intended and what I did
Visually, the biggest change between the old and new
Progression is more compact and elegant internal sinks thatlessen chassis weight by 10lb. The width is virtually thesame, but the M550 is 1.5" longer and 3" taller. The power
on/standby switch remains hidden on the bottom front

measurements, continued

As with the earlier amplifier, I
examined how the THD+N percentage
varied with frequency at a high level,
28.3V, which is equivalent to 100W
into 8 ohms, 200W into 4 ohms, and400W into 2 ohms. (The front-panel
meter’s needle lay just above “500”at this level.) The results are shown in
fig.6: The distortion and noise remain
low over most of the audioband into
the higher impedances with only aminuscule rise in the top octave. (Thisgraph confirms the test results sent by
the manufacturer.) However, when I examined the waveform of the THD+N at
100W into 8 ohms after notching out
the fundamental (fig.7), I saw spikes in
the residual waveform (bottom trace)that coincided with the zero-crossing
points in the signal’s waveform (toptrace). This behavior indicates thepresence of crossover distortion.
Fig.8 shows the spectrum of theamplifier’s output while it drove 50Hz
at 100W into 8 ohms. The subjectively
innocuous second harmonic lies at–74dB (0.015%), but a regular seriesof high-order harmonics can be seen,
which correlates with the waveformof the spuriae shown in fig.7. The harmonic spectrum was very similar withthe amplifier driving 50Hz into 4 ohms
and 1kHz into 8 ohms at 28.3V (notshown). At 1W into 8 ohms, the second
harmonic was still the highest in level
but lay at –60dB (0.1%, not shown) andwas still accompanied by higher-orderharmonics. Fig.9 shows the spectrumof the M550’s output as it drove an
equal mix of 19 and 20kHz tones into
8 ohms at a peak level of 100W. Whilethe difference product at 1kHz lay at an
okay –73dB (0.016%), there are also
many higher-order intermodulation
products visible in this graph.
The Dan D’Agostino Master Audio
Systems Progression M550 is a powerful amplifier indeed and will not be
fazed by being required to drive lowimpedance loudspeakers. However,the presence of what appeared to be
crossover distortion, which suggests
insufficient output-stage bias, is puzzling.—John Atkinson

d
B
r

d
B
r

A

A

Avg: 32

Hz

Fig.7 Dan D’Agostino Progression M550, 1kHz
waveform at 100W into 8 ohms, 0.062% THD+N(top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

stereophile.com n November 2021

Fig.8 Dan D’Agostino Progression M550, spectrumof 50Hz sinewave, DC–1kHz, at 100W into 8 ohms
(linear frequency scale).

Hz

Fig.9 Dan D’Agostino Progression M550, HF intermodulation spectrum, DC–30kHz, 19+20kHz at
100W peak into 8 ohms (linear frequency scale).

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D’AGOSTINO PROGRESSION M550

edge, below the large power meter
which has adjustable and defeatablegreen illumination, and the rear panel
contains the same 20A power inlet,
main-power toggle switch, XLR input,
12V trigger, plus binding posts of different design. The feet are still big, round,gray, easy-to-remove rubber thingees.
When Bill McKiegan, Dan
D’Agostino Master Audio Systems
president and Dan’s long-trusted associate, arrived to help set up the M550s,
we placed them on the Grand Prix
Monza stands I used for the original
Progressions and attached the same
Nordost Odin 2 20-amp power cables,
XLR interconnects, and speaker cables.
I’d previously had long discussions
with Dan about power and knew from
Timpani sounded I discerned a sly change in tonal color thatprior experience that, in my system,
had never come through before—nor had I
clean and strong,amps sound best when plugged into thenoticed places where RLJ intentionally lags aand the lowesthigh-current outlets of my AudioQuest
bit behind the beat.
Dragon-powered Niagara 7000 power
On the fabulous Catherine Russell andpitched lines ofconditioner.
Wynton Marsalis take of “Make Me a Pallet
the double basses on the Floor,” arranged by Marsalis for theAfter a brief listen, Bill encouraged
me to try setting up the Progression
movie Bolden (24/96 FLAC, Qobuz), I heard
were clear, solid,
M550s and Momentum HD premore subtlety and shading to the cello—moreand substantial.
of the stuff that distinguishes a rendition as
amp the same way they’re set up at
special. On a very different recording, of
D’Agostino’s Arizona factory, with thethe significantly higher-voiced and radiant Elly Ameling
electronics resting on their attached rubber feet. I’d always
sparkling her way through Schubert’s “Die Sterne” (Thepreferred the effects of aftermarket supports to those rubber
Stars) with Dalton Baldwin on piano, from The Artistryfeet, but those that sounded best had been discontinued andof Elly Ameling (Tidal and Qobuz 16/44.1 FLAC, Philips
returned to the distributor. I was curious to learn if return473-4512), the voice floated in space like I’d never heard it
ing to the rubber feet might help address some longstanding
before, and the bass line of the accompaniment, which isn’t
issues in my system.
superbly recorded, was clearer. Similar distinctions of detail
During my first listens after Bill departed, I discoveredthat the lower midrange had lost its customary focus. Think- and presentation surfaced on Mahler’s anything-but-subtle
Symphony No.3 performed by Iván Fischer and the Budaing back to my chats, at T.H.E Show 2021 in Long Beach,with Norman D. Varney of A/V RoomService, I decided
pest Festival Orchestra (Channel Classics DSD128). On thisto try his EVP (Equipment Vibration Protectors) under therecording, pitches were clearer on divided horns, and theProgression M550s. Varney, who subsequently paid a visit to“burr” on horns was more discernable. Timpani soundedPort Townsend after spending time at NWAA Labs in Elma, clean and strong, and the lowest-pitched lines of the double
Washington, designed the electrical grounding/isolation
basses were clear, solid, and substantial.
setup for George Lucas’s Skywalker Ranch Scoring Studio
Another big ear-opener arrived when I transitioned
and touts accredited acoustic lab measurements to back up
between files of two recently reviewed DXD recordings:
his claim of the positive effects of his EVP’s effectiveness at
Sono Luminus’s Northscapes from pianist Ieva Jokubaviciute
isolating equipment from vibration.
and 2L’s Solacium from Trio Mediaeval. The latter is enThe A/V RoomService EVPs, together with the HRS
coded in MQA, but I believe that different environments,
DPX-14545 Damping Plates I’d used on the original Promicrophone placement, and engineering accounted for
2L’s markedly lower noisefloor. I listened intently whengressions, did more than resolve all lower midrange mush;
they also helped smooth out some longstanding rough edges I reviewed both these recordings with the original Proin my system without adding brightness, glare, or other colgressions, but I’d never heard such huge distinctions inbackground noise.
oration. I’ve continued to experiment with ideal placement
The sound of the new monoblocks was in the same famof the EVPs, which I use on many components, digital andanalog, that are not supported by Wilson Audio Pedestals.
ily as the original Progressions but warmer, with a subtle
glow and smoother finish that produced rounder images.
There was an unmistakable similarity to the beautifully finTime for listening
Once those setup issues were resolved, I was a happy campished sound of the other amplifiers I’ve reviewed with either
pure class-A or significant amounts of power in class-A,
er as I noticed at once the M550’s ability to convey subtle
notably the Pass Labs XA 200.8 monoblocks, Gryphon Esdetails. In-breaths were more evident on Rickie Lee Jones’s
marvelous take of the Stones’ “Sympathy for the Devil”
sence monoblocks, Krell K-300i integrated, Audio Research(Tidal, 16/44.1 FLAC), from her album The Devil You Know,
REF 6SE, and Doshi Evolution monoblocks. You may not
and the transition from soft singing to whisper was more
be surprised to learn that these are the best-sounding amps
impactful. On the line, “I killed the Czar and his ministers,”
I’ve reviewed over the past half-decade.
stereophile.com n November 2021

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D’AGOSTINO PROGRESSION M550

When I reviewed the relatively low-powered Gryphon
Essence monoblocks, the recording that allowed themto steal my heart was the late Lorraine Hunt Lieberson’s
mesmerizing live rendition, with pianist Roger Vignoles,of Mahler’s “Ich Bin Der Welt Abhanden Gekommen” (“Iam lost to the world”) from the Five Rückert-Lieder, as preserved on Lorraine Hunt Lieberson: Mahler, Handel & Lieberson(Wigmore Hall Live, Tidal 16/44.1 FLAC). It’s a remarkable recording—one that makes Wigmore Hall sound muchbetter than the two times I sat in the hall’s less-than-prime
seats during a 2019 trip abroad.
Listening through the D’Agostino Progression M550
monoblocks, what impressed me most (besides the beautyof Hunt Lieberson’s voice and her total identification withmusic and text) was the Progression M550s’ ability to convey the inner warmth and depth of someone transported.
I was equally struck by the clarity of a piano recorded ina surprisingly quiet and exceptionally dimensional acoustic. The silence was as astounding as the heart that shone
through this great artist’s voice. When an amp can deliver allthat from a simple performance with voice and piano, it’s agreat amp.
Progress(ion) report
With my assessment presumably complete, I implored husband David and audiophile buddy Scott Campbell to help
me move monoblocks back and forth for direct comparisons.
On my trusty deep bass electronically hyped standby,
Boris Blank’s “Electrified II” from Toy (24/48 MQA FLAC,
Tidal), the original Progressions sounded splashier on Malia’s vocals. There was less color differentiation, soundstaging was less awesome, and the presentation seemed plainer,
flatter, and less muscular and dynamic. Returning to Elly
Ameling’s smaller-scale recording, her voice seemed to lose
some warmth and radiance. There was less silence aroundthe voice and less three-dimensionality. Listening was lessinvolving.
One of my go-to recordings for color, depth, microscopic
detail, expansiveness, and dynamic contrast is Anna Thorvaldsdóttir’s Metacosmos, performed by the Iceland Symphony Orchestra on Sono Luminus’s Concurrence (24/352.8
WAV, DSL-92237). Thorvaldsdóttir places great significanceon the emotional impact of a single instrument, briefly singing its own song like a bird in the distance as many dozens
of others issue their own calls. The original Progressions
smudged such details with clatter, occasionally burying thementirely. When everything started going at once at higher
volume, they struggled to separate one instrument from theother. They also sounded less rich, with less gratifying inner
warmth and contrasting colors.
In addition to their far blacker space between notes,
greater resolution of small details, and natural-sounding
illumination, the Progression M550s consistently wowed mewith their dynamic mastery. Early in Metacosmos, where
low bass rumbles are suddenly interrupted by a loud, sharp
crackle, the familiar entry was so powerful that it startled
me. Later, when every instrument and its mother started
playing at once, the M550s nailed the brutal force and savagery like no other amp I’ve ever heard in my system. The
original Progressions struggled to separate one instrumentfrom the other, and small details were buried amidst the din.
I was shocked by the cumulative impact of the differences
between old and new Progressions.
58

A S S O C I AT E D E Q U I P M E N T
Digital sources dCS Rossini DAC and Clock and Transport;
Synology 5-bay 1019+ NAS, Roon Nucleus+ music server,
Uptone Audio etherRegen, Small Green Computer Sonore
opticalModules (2) and Deluxe opticalModule, Linksysrouters (2), and Arris modem, all powered by HDPlex 300
four-component linear power supplies (3) and HDPlex 200
LPS; Apple 2017 iPad Pro and 2017 Macbook Pro laptop
with 2.8 GHz Intel i7, SSD, 16GB RAM.
Preamplifier Dan D’Agostino Momentum HD.
Power amplifiers Dan D’Agostino Progression monoblocks.
Loudspeakers Wilson Audio Specialties Alexia 2 with
Acoustic Diode Spikes.
Cables Digital: Nordost Odin 1, Odin 2, and Valhalla 2(USB); Frey 2 (USB adapter); Wireworld Platinum Starlight
Cat8 (Ethernet) and OM1 62.5/125 multimode duplex
fiberoptic cables. Interconnect: Nordost Odin 2. Speaker:
Nordost Odin 2. AC: Nordost Odin 2, AudioQuest Dragon,
Dragon HC, and Thunderbird. Umbilical cords from HDPlex
LPSs: Ghent Audio Canare and copper.
Accessories Grand Prix Monza 8-shelf double rack andamp stands, 1.5” Formula platform, Apex footers; Symposium Ultra Platform; Nordost QB8, QX4 (2), QK1, and QV2
AC power accessories, QKore 1, 3, and 6 with QKore Wires,
Titanium and Bronze Sort Kones, Sort Lifts; AudioQuest
Niagara 7000 and 5000 power conditioners, NRG Edison
outlets, JitterBugs; Tweek Geek Dark Matter Stealthpower conditioner with High Fidelity and Furutech options;Ansuz Darkz T2S resonance support feet; Wilson Audio
Pedestals; A/V RoomService EVP Equipment Vibration
Protectors; Resolution Acoustics and Stillpoints Clouds(8) and Aperture 1 (2) and 2 (2) acoustic treatments; HRS
DPX-14545 Damping Plates; Stein Music Blue Suns, Blue
Diamonds, and Quantum Organizer; Bybee Room Neutralizers; Absolare Stabilians; Marigo Aida CD mat.
Room 20' L × 16' W × 9' H.—Jason Victor Serinus

From great to “gaaah!”With the Progression M550s back in place, all notes taken,
and my listening presumably complete, I set about enjoying
myself. Everything was smooth sailing when Daryl Wilson
emailed that he finally had enough stock of the new Wilson
Audio Acoustic Diode Spikes ($3200/set of eight or $3000
when ordered with current speaker models) to send some totry under the Alexia 2s. The Acoustic Diodes (AD Spikes),which come standard on Wilson’s Alexx V loudspeakers, were designed with the same laser vibrometer testing
equipment that was employed in the development of the
Wilson Pedestal equipment supports and benefited from theresearch that was essential for the Pedestal’s creation.
I then enlisted the assistance of Gary Bruestle, one ofDefinitive Audio’s speaker setup wizards, to help attach
Acoustic Diodes to the Alexia 2s. Since that required releveling the speakers, I felt that the significant changes I’d madeto electronics, acoustic treatment, rack, and supports since
the Alexia 2s arrived years ago mandated a reassessment of
speaker position.
Keeping the speakers in their original position, we installed the Acoustic Diodes. The soundstage widened, detailand texture became more discernable, and bass grew clearer
and more defined. The impact of better amplification andNovember 2021 n stereophile.com

D’AGOSTINO PROGRESSION M550

superior loudspeaker spikes
became evident on the Kronos
Quartet’s performance ofTerry Riley’s Sun Rings (24/96,
Nonesuch 587972), which
I wrote about for My Back
Pages in the August 2020 issue.
Even though I thought therecording pretty damn special
to begin with, I heard a wider
and more coherent soundstage
coupled with more far moredetail, layering, and silence. At
the risk of sounding like aninfatuated teenager, the soundwas awesome. No wonder Leslie Ann Jones and her fellow
engineers won a Grammy forthis recording.
Next, we repositioned the speakers several inches closer
to room center while keeping them the same distance fromthe front wall. And that’s when we discovered that left andright channels sounded different from each other. From theleft came warm, color-saturated, extended sound; the rightsounded surprisingly flat and rolled off on top. Given thelarge opening to the hallway on the left side of the music
room, I’d expected some differences between channels, andmeasured the same during my Accuphase DG-68 Digital
Voicing Equalizer review. Was that what I was hearing,
except far more clearly now that the speakers had been
repositioned?
After lots of cable and channel swapping, and many left/right balance checks while playing part of Hindemith’sSymphonic Metamorphosis on a brightly recorded Mercury
Living Presence monaural CD compilation featuring Antal
Dorati, Rafael Kubelik, and the Chicago Symphony Orchestra (16/44.1 WAV, ripped from Mercury 289 534 397-2),
the evidence was incontrovertible: I had two very different sounding M550s. As confident as I was that the more
extended and tonally rich amp had determined the majorityof what I’d heard during my evaluations, something wasclearly amiss.
Following late-night phone consultations with Editor Jim
Austin, I awoke to speak with Bill and Dan about the possibility of delaying the review and sending the amps back tostereophile.com n November 2021

Arizona for repair. Dan, though, was certain that the amps
were fundamentally sound. He thought the discrepancy wascaused by a shift in biasing, which presumably happened
during shipment.
Two tension-fraught days later, Hans Brackmann, Definitive Audio’s store manager, drove to Port Townsend to rebias
the amps. All D’Agostino dealers are prepared to rebias theM550s whenever necessary.
Sure enough, rebiasing was called for—just that, nothing
more. By the time Hans left, both channels sounded thesame (save for minor discrepancies caused by room imbalances), and I heard even more of the beautiful, smooth,finely detailed, impeccably controlled, remarkably dynamic,slightly warm and glowing sound that had made me fall inlove with the Progression M550s.
So.
Once the Progression M550s were optimally set up andbiased, love at first listen morphed from an infatuation intoa relationship.
For anyone who values colorful and glowing amplification that brings natural timbre and the subtlest of details anddynamic shifts to the fore while supplying a breathtakingly
full measure of big-picture dynamics, slam, and top-tobottom frequency response, the Progression M550s must be
heard. They are wonderful amps. n

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