Hey all. Just joined up.
I am a serious audiophile (and electrical engineer), and one of my primary reasons for a smartphone to begin with was to replace my aging Cowon D2 (hacked to D2+). Despite this, I didn't really look into how the sound quality was going to turn out before I got the Thunderbolt.
Unfortunately, my Shure SE420 in-ear monitors have background hiss whenever audio is active at any volume that I can absolutely not tolerate. However, I have a bit of good news: there is a fix for this.
Essentially, the HTC headphone amp is set up to give you the highest volume instead of the best quality output, and this results in the hiss. This can be solved by increasing the impedance of the headphones you are using, or getting headphones with a higher impedance.
My Shure IEMs have an impedance of 22 ohms. The hiss is unbearable for me.
I also have a set of closed-back over-ear headphones, AKG K271 MKII, which have an impedance of 55 ohms, through which I can't hear any hiss at all.
Obviously, I don't want to pack around some huge headphones, so I will need to increase the impedance of the Shure IEMs. This can be done by adding a 75 ohm resistor into the wire for each channel. This can be a $5 DIY fix, but if you aren't interested in that, or if you don't have time, you can get a high-quality pre-made one
here. Unfortunately, they are around $30 with shipping. You can also look at a different one
here.
Changing the impedance of your phones
will change the character of their sound a bit (possibly for the better - YMMV - but I won't go into that, but you can google about it - headfi.org is an okay resource, but they do deal in a lot of snake oil as well). I intend to use the PowerAMP app with the equalizer to tailor the sound to my liking, but from other's experience with my particular headphones, the sound character
should be changed for the better with the increased impedance.