[Samsung Galaxy S8 Questions and Answers] Paradoxxx:
Let's compare scientifically gathered numbers here :

Apple iPhone 6s (headphones attached)	+0.10, -0.06	-93.8	93.9	0.0030	0.101	-68.2

LG G5 (headphones)	+0.05, -0.01	-92.2	92.3	0.0029	0.037	-50.7

Samsung Galaxy S8+ (headphones attached)	+0.03, -0.03	-92.5	92.5	0.0024	0.046	-77.3

And to quote GSM Arena :

"Stellar audio output

The Samsung Galaxy S8 matched its Plus sibling for clarity, delivering perfectly accurate output both with an active external amplifier and headphones. Even the increase in stereo crosstalk with headphones was barely noticeable and exceeding our expectations.

Somewhat surprisingly the smaller member of the flagship duo outdid its sibling in terms of loudness. The Plus was not quite itself, but the Galaxy S8 is among the loudest handsets out there, rounding up a great performance."

Let's see the frequency response now :

iPhone 6S :

LG G5 :

Galaxy S8 :

As we can see here, LG G5 is the worse of the pack, with a cut-off way before S8 and iPhone even if it has the flattest frequency response. However such a different is not audible by human ears.

The iPhone has quite a wobbly frequency response, but manages to have a high frequency cut-off.

The Galaxy S8 is beating both of them, with almost perfectly flat frequency response and higher frequency cut-off, but it has a slight wobble across the whole frequency which makes it less studio-like than the G5.

Noise level wise, the iPhone is the clear winner, with a dB more than the others.

Dynamic Range, the iPhone is again the winner, with more than a point better than the two others, with the G5 being the worse.

Now comes the interesting part, harmonic distortion. For these who don't know what THD is :

"Total harmonic distortion (THD) and the inter-modulation distortion (IMD) measure the unwanted altering of the original audio signal. When an audio signal passes through a non-ideal, non-linear device (such as your smartphone), additional content is added to the original frequencies in the form of interference, which results in distortion. Distortion is the name given to anything that alters a pure input signal in any way. So, as you might have guessed, the lower the number in either of these fields, the better the audio output of your device."

The galaxy S8 has a THD of 0.0024, being the best among all the devices previously mentioned.

The stereo cross-talk on the iPhone is absolutely horrible, while the G5 and the S8 maintain a score close to 93.

I could compare other phones, but I'll just add one more, the One Plus 5 :

OnePlus 5 (headphones attached)	+0.15, -0.08	-94.0	94.0	0.0033	0.139	-59.9

We can see that this phone has actually a worse frequency response than the phones we were taking about today, with, however a better noise level, and dynamic range than the others but worse THD and stereo crosstalk. Therefore it might not be the best when it comes to reproduce your song to the way it was actually recorded (even though if +0.15 and -0.08 across the whole range is probably not that noticeable) but it goes louder, and the sound differential is actually bigger, which could give a better feeling of quality than the two others.

So to make a long story short, and hopefully close this topic once and for all, no, the S8 does not have a worse output than the previously mentioned phones. It does NOT have muddy output, or over-powered bass. It has an excellent stereo cross-talk and the lowest Harmonic Distortion across the board. It's probably the best phone to reproduce your songs the way they are meant to be.

So why someone's opinion is different than the actual scientific results ?

Some may say placebo, but I tend more to think that it's related on how we actually listen to music, our equipment, and possible ear damage.

I have a pair of IE-80 which are quite high-end Sennheiser IEMs. They sound fantastic with my S8, however I cannot stand any stock sound anymore. Not because the stock output is bad, and as we've seen previously it is far from being so, it's just that I listen to a lot of EDM songs, and I like my bass a little punchier than what the stock output provides, or that any other phone provides for that matter. So I'm using ARISE on all my devices. Regarding the max noise level, I have a custom mixer_gain.xml that I'm using on my S8 which allows for much louder output, with the only downside is to add a bit more noise on quiet songs. But that's the way I enjoy my music.

If you really want to know more I invite anyone that is interested to check GSM Arena analysis, and compare the numbers on phones you might be interested in purchasing.

Hope this ends the "this phone has a terrible aux audio" story.

Peace !