poor vocals on headphones, ok on speakers

[LibR8R]

I've only just got my Zune 80gb . Some tracks when i listen to them over in-ear headphones & i've tried 6 pairs now incl bose, have the vocals almost totaly lost in the rest of the sound, but if i play the same track over a set of cheap travel speakers the vocals are clear. The vocals were clear when i play the track on the laptop & on my old mp3 player. the music is mostly indie-rock, any ideas?
why this might be happening & how to bring the vocals out more.