I don't have a hiss issue at any (and all) volume levels (with player on music off) or with music on. There is only an ever so slight sound you can hear between the time you stop a song and a little click (3 seconds later). It is not a function of volume, however. In fact, standing in the kitchen, I have to move away from the refrigerator to even hear it at all.
What model S2 do you have? Do you have the I9100? If so, we don't have the same phone. I have the SGH-T989. I'm sorry about your hiss issue. Some have reported actually hearing the CPU. I can't imagine what that sounds like.
On the T989, the CPU is a 1.5 MHz dual-core processor and the battery is 1850mA. I work a 12 hour shift and place/receive phone calls (via bluetooth), listen to FLAC songs (for about 4 or 5 hours) and turn the screen on a lot (all day) and I always get thru the day. The battery is usually around 20% when I get home. If I had a battery problem, I'd buy a 3600mA battery with back-cover (to accommodate the added girth) for just 20 dollars. It'd be a mini-brick, but you could do whatever the heck you wanted (all day long) without concern.
About the messy rendition of some rock songs, did you up the Poweramp priority and buffer size? It's also good to close as many background apps (used by Android) as possible. And kill that Music app, because it opens itself when you start a song.
I don't have any technical knowledge of how Poweramp (and a few others software players) manage to get better results from the hardware than the standard app. I don't know how RockBox transforms the Sansa Clip+, but it does. The difference in sound is undeniable. If there's an ever so slight level of distortion added, it is negligible (and on the go, forget about it).
As for the amps, the Headstage Arrow HE 4G is $300 (and a long wait). It has nice features like turning itself on (and off). The battery life is ridiculously good. I don't even know the times between charges. It's like a iPod in that regard. It's steady rather than terribly powerful. And no, I don't have a particular configuration. It's just a matter of what I feel like listening too. The big headphones and small amps (and non S2 players) all play nice with one another. The S2 and 535s only like one another exclusively. They hate everyone else.
The FiiO E11 (at $60) has slightly more punch that the Arrow, but runs out of gas much quicker. Maybe 4 or 5 hours at high gain (maybe not even that long). You can easily forget to turn it off (in which case you runs out of juice). It's less clear/clean than the Arrow.
The Electric Avenue is the oldest and heaviest of my few little amps. I haven't used it in a long time though it works perfectly. Years ago, a basshead co-worker with an 18in woofer in his SUV told me if I turned the volume all the way up on my little headphones (AKG K26P), he would NOT be able to hear the bass. So, I plugged them into my Dell Axim X51v pocket PC (my source unit at the time) and the EA amp (playing a hip-hop song with bass). He snatched the K26Ps off his hears and shouted; “DAMN!! That thing was rumbling on my head!” He learned a deafening lesson about SPL at close range. IEMs and a wide open volume knob in my pocket? NO THANK YOU! I'd like to be able to hear the grand-kids when I get older.
The FiiO and EA amps hiss like crazy with sensitive IEMs. Someone reminded me of that on another thread. The Arrow hisses as well but not so much at low (actual listening) volumes.
I can't imagine putting an amp between my S2 and my 535s. I can't turn the volume up much past half-way as is. I've got plenty of bass (for non-Rap music) with the bass tone knob at halfway and treble at halfway as well.
Super low sub-bass is probably a technical (physical) issue (with one BA less capable than one dynamic speaker). There are sub-bass notes in songs (that I know are there) that my Cowon iA9 won't even pretend to reproduce (to even deliver to an amp and headphone).