[Xperia Z1 Themes and Apps] barkohba:
Ok so here's the deal with (truly) increasing the ANALOG (which is what you want) volume of your xperia.

For starters, I want to specify that what I am about to present is valid, to my knowledge, to Xperia Z1, STOCK (I don't know how it behaves for other firmwares like CM11) and only rooted.

Also, I am using an MDR-1R (Sony) headset. These are high end over the ear headsets, and what's important to note is that they're made to handle, among a bunch of other sound details, high volumes without distortion,

HPH vs RX (digital):

If you increase the RX volumes, you will get a digital boost. What that means is basically you WILL get a volume boost, but that boost comes with the cost of quality, and will clearly deteriorate the sound, making the sound feel very uncontrolled and distorted. That is not normal at these volume levels with the headsets that I use, due to the fact that when I connect them to an iMac (which has a much more powerful and higher volume than what can be achieved on a smartphone/mp3 player due to its high quality sound card) the sound keeps all of its details intact, with NO distortion.

The HPH (L/R) is what you're after. This increases the analog volume (aka true volume), which raises all of the frequencies equally, giving you a real volume boost, without distorting the sound (if you have a low end headset, at high volumes the sound could sound distorted not due to the phone, but because of the headset drivers/quality). For reasons I can not explain, raising the value above 20 (I actually tried it with 23 and 25, but I have the feeling even at 21 the same thing happens, though you are free to try and report on your findings) makes the volume drop way below standard. So what you want to do is increase both HPH levels to 20!

Now in my case, and this is valid for high end headphones (at least to my knowledge), setting the HPH to 20 and slightly increasing the RX to 85 helps a bit, as the lows and especially the mids get a small boost without distorting the sound, making my MDR-1Rs sound as good as possible on a mobile device without a portable amplifier.

Basically, for me, with the aforementioned headset, the best settings is HPH-20 (both), RX-85 (both) .

You can fell free to play around with these settings, but the bottom line is, for most headsets, especially the in ear ones that come bundled and other low or mid range headsets, the best settings is increasing the HPH volumes up to 20 (or less if at max volume the headsets give out uncontrolled and distorted sound), and completely leaving the RX alone, as increasing it to levels above 85 basically destroys the sound quality, and we don't want that.

Of course, if you have high quality over the ear headsets, which tend to have pretty low volume on mobile devices due to their large drivers which need a lot of power, the absolute best option to get the most out of them is buying a mobile amplifier. Of course such amplifier cost around 600 dollars, and are specifically targeted for rich audiophiles or musicians. Sony makes on of the best portable amplifiers out there (the PHA1 - details here https://pro.sony.com/bbsc/ssr/cat-audio/cat-headphones/product-PHA1/ ), but it's an expensive proposition since it costs as much as a flagship xperia, and is even a bit larger than one (though it does come with a special straps to hang on to your device)

Hope that helped!