After the recommendations in this thread I got the unlocked version of PowerAmp.
1) Poweramp has a lot of background hiss, I am using Shure 535 straight out of the jack, and I can hear it easily in the quieter sections.
2) Gapless playback is poor. C'mon guys, my iRiver H320 had great gapless playback 9 years ago! Even the iPods, after Apple dithered for a year or three, got gapless playback 6 years ago.*
3) Handling the other sounds from the phone while playing music leaves something to be desired, sometimes it does a good job, other times it stays on a lower sound level for several seconds after the notification sound has finished. Sometimes it pauses and I have to push play to get it going again.
4) The lock screen controls are good, but it often stuffs up the album art. Sometimes the album art is displayed, sometimes it isnt, sometimes it appears only to disappear a second or so later.
I dont know how Poweramp got such a good reputation, it must do a better job on other platforms?
<rant: anti fanboi>
I remember the Apple Fanbois defending Apple at the time: "but there are meant to be gaps between the songs" and "just use crossfade, it's the same thing". When iRiver came out with the H320 they said things like "who would want a colour screen on an mp3 player anyway". Until the next iPod came out with a colour screen and then it was "Apple leads the way, again" despite them being 6 months behind.
1) The hiss you hear via your sensitive 535s is an android/samsung thing, has nothing to do with poweramp.
2) Gapless playback works perfectly for me. Make sure cut silence at beginning and and of the songs is off.
3) Make sure "Short Audio Focus" is checked, this will pause playback when an external notification sound is played. Playback will resume promptly at the right volume when the notification sound is done.
4) I agree the lock screen implementation is pretty bad, I just use the default android (ICS) lock screen instead.