Just plugged my Cowon into my CA integrated, straight from the headphone out using a cable that couldnt have cost more than $2 to make, plugged the 601s into the headphone jack on the CA and cranked up Deftones' 'Hole in the Earth'. Pure sonic bliss, complete with great separation between the kick drums and cymbals, something I don't get with the MS-1s (not to mention soundstage, something my newbie ears are still coming to grips with). As I've said before, anything less than high bit-rate tunes (FLAC/Lossless/320Kvbr) married with adequate amplification is going to be like listening to a transistor radio 3 doors down. Under a blanket.
A better source and correspondingly better amp would only improve the 601s, IMO. Apologies for not having empirical evidence to back that claim, but I've been pretty hard on AKG of late - maybe I just need to buy the 5K worth of kit that will do them justice and shut the hell up .....