Speaker / headphones sound quality is only as good as the source. If you think Sansa Fuze sounds like crap, then you will not notice any improvement.
In fact, audiophile level equipment (speakers, amps, etc) will make crappy quality sound even worse because these type of equipment can reveal all the flaws. Note, those headphones are not considered audiophile.
Cowon is known for selling players with the best sound quality, specifically the D2+ and S9. I plan on picking up the 32GB version of the S9. Too bad there is no 64GB version and there probably will never be. They support video playback though, but many reviews (not just personal reviews) proclaim them to have the best sound quality; the S9's SNR is 95, not sure of the D2+.