i think that muddiness is in the ears of the beholder. i had the cowon d2 which i found with trance music or music with hard fast bass to have a small and closed soundstage that got muddy very easily.
both the 3g and the d2 hiss but very little compared with other players i have tried. i own the atrio m5, the denon c700, beyerdynammic dt770 and dt880.
i used to own the westone um2 and er4s and spent heaps of time with the e500 - all of which i did not like on the d2 though the westone seemed to be the most appealing out of those.
for large phones, i find the nano 3g is not the best player but for small phones, it is one of my favourites. it gives great soundstage for trance and good placement as well as speed. i get none of the muddiness that i heard in the d2.
however, as i said, muddiness is in the eyes of the beholder. other than muddiness and soundstage, i found the d2 and nano 3g to sound very similar with the same phones. both are detailed and have little hiss with good bass response considering neither can properly drive low imp phones.
i own the sony a615s as well - great sounding player for low imp phones. i find it lacks stage and overall satisfaction (look out for the technical terms here) with my big phones.
in terms of satisfaction:
road:
nano (bus)
walk: just want to use tactile controls and have a player whose build quality and looks wont be bothered by a drop (if it drops, it would probably crack or something - quite flimsy but it is cheap)
SONY
home: with big headphones the others do not satisfy like the
cowon does. no muddiness anymore, great soundstage for a portable and without an amp, completely forgiveable.
with an amp however, the ipod again takes stage. i was surprised how much i liked it after all the negativity here at headfi - perhaps that is why i should have liked it! it is not trendy to like ipod
EDIT: cowon d3 is not yet out: perhaps the poster in the first page meant cowon d2. d3 will probably be out by june - if indeed it will be called d3.
the zune 1 and nano 3g use a very similar dac. the headphone out on that unit is basically also the headphone out and then the only real extra are caps for output power. most likely both the zune and ipod use only single end caps leading to the massive bass rolloff but i find it not too bad on my nano 3g... just not huge visceral impact. the documentation about this can be found
here.