it depends on the song... for instance, metallicas cd... what was it... Death Magnetic (had to google it) the cd version of the songs is probably better heard on mp3, at least that way you can delude yourself saying its bad because of the mp3, not because of how it was mixed.
no quality of headphones is going to fix that.
but than you have things like classical music, which will benefit immensely from higher quality recordings.
point being, not all music will suddenly be better because alot of whats out now is made for the crap headphones, and the crap players, so they dont take the extra time that they would have years ago to master it perfect.