output. on the c700's
zune or ipod depending on what interface and usability you prefer.
If you want to eq grab a 30g ipod from somewhere and rockbox it.
The advantage is later if you want an amp instead of buying a new player you can diymod it or imod it.
did you want wifi on the player? if so u need a pmp ...
sound quality is subjective. personally I'd reccomend the sony's for amazing sound quality - a bit of hiss and harsh high's but max 16gb
a lot of people here like the cowon d2...but the sound is noted for bass rolloff and lots of fiddling to get the right signature...much like the 30gb ipod with rockbox minus the rolloff..but the ipods are noted as having pretty bad headphone out.
if you dont like playing around grab the sony if you dont mind the 16gb otherwise grab zune/ipod ez no fiddling lots of room and reputable.
edit : try to find some loaded player results with an ipod or zune with your particular headphones...will help your decision