I've recevied my K601 (#1644), and have been listening to them for a very long time now (Almost can't take them off).
I'm listening through an iRiver H332 and a Corda XXS amplifier, and have also tried it with a 2nd gen touch through a LOD and a Cowon D2.
Listening to Jazz and Classical with these phones were spine tingling. For example, listening to "cry me a river" by Diana Krall, the soundstage was excellent, with very detailed vocals. The guitar solo bit in the song was especially mind-blowing. Trying something more classical such as Beethoven (I've been listening to classical a lot lately as it helps me concentrate when studying), again the sound was excellent even though the treble is a bit recessed.
What I find lacking is Bass, as listening to something faster such as Rage Against the machine and Audioslave revealed that they lacked that nice, punchy character of my Grado SR60, which I think I may keep around as I love them for rock. Obviously, the SR60's vocals aren't on par with the K601 IMHO, but I find the K601 much more comfortable to listen to over long periods of time, something that I couldn't say for my very bright SR60. Replacing the iRiver with my 2nd gen touch seemed to improve matters, but then the volume through LOD was way too loud for my ears to be comfortable, and turning down the volume too much on the XXS induces channel imbalance problems. The D2 sounded horrible with the K601 and XXS though, way too neutral and lacking in depth for my liking.
I find the K601 very comfortable to wear, if a bit on the heavy side
They were much bigger than I was expecting. Hooking it up to my CLip + FiiOP combo, I was surprised at the sound. Good, if a bit bloaty bass, but the treble was a bit rubbish. Then again, it was only for fun.
I have a feeling that my XXS is holding these monster cans back. Time for a home amp I think, but the question is which one???
EDIT: Switching over to my Soundrigor amplifier, there is much more Bass, but its a bit bloady and "aritificial". Also, the trble doesn't seem as nice as the XXS.