I have to admit I definitely listen at a lower volume. I have my volume limit on my iPhone4 set to about 50-60% and then in EQu or Equalizer (use both) I have my volume 75%-100%. There are a few times I'd bump my volume limiter up a little bit, but I certainly don't like to have the volume cranked. When I listen at 30-60% overall volume output (considering full volume range without limit on the phone), I should be able to hear all the details of the music just the same as if it were cranked at 80% volume, just less painful ;)
edit: after thought: What I would really like to know and need to do some more research and find a local store where I can try it out, is with an Amp, can I maintain my listening volume, bring out more soundstage and fuller sound overall (such as with the eq'ing), but without the eq'ing. Like I said though, all more research and trying for myself :)
I believe that is exactly what certain amps do based on reviews and impressions I have read. In-fact, one of the main sought after or commendable features of certain high end amps is widening the sound stage and improving clarity, bass impact etc. Having said that, I'm not sure if they would have such drastic results as EQ'ing, but you never know. I am still very new to the world of amps, but like I said, I plan to do a lot more testing with different amps over the coming weeks/months.