Volume on my SGP5.0 is pretty decent. Netflix is fine with volume 20-30. If I am outside, that's another story. Sound is dispersed and much more volume is needed.
I really would not put more volume into my earbuds (Klipsh S4)-30 is WAY too loud! I'm not a classical purist wither; I've got a bunch of Zappa, Zep, Aerosmith, Floyd as well as a lot of blues, indie, jazz and classical. I've been to may shows and sat near the stacks in the 70's and 80's...With my 5.0, Never felt like I needed to get more volume. My ears would be wrecked! And if you're going into a PA, stereo or another amplifier, setting the output to somewhere between 20-30 drives most amps front ends just fine.
I have not used anything other than the stock music player but it seems fine. I do notice that compared to my fuze version 1 the sound is more flat, without the same presence or detail. But of course this is stuff that you only really notice when A-B'ing side by side. And after all, we're talking about mp3 files here for the most part, which obviously degrade sq.
And absolutely no hiss for me into my S4's with volume at 30.