I suspect a major issue with headphones and small players is that the =
tiny amplifiers just cannot cope with the power requirements of drivers =
bigger than the ultra-small earphone ones.
If you are not averse to carrying a small bag with your portable gear, =
and you want to use full sized headphones rather than earphones, then I =
recommend a browse through
www.headphone.com.
I have one of their (non-portable) headroom amps, and the extra power =
and the crossfeed circuit improve the sound from my Sennheiser =
headphones quite remarkably.
Greg