I just realized my post was pretty jerk-ish, so I apologize for that, but I definitely feel the CD900STs really are in a separate league as far as mids go. (Getting a little off topic, but I never miss a chance to praise the CD900STs ) They are just really picky about amps, so on the setup you heard them through it's possible that they fell behind the K501s because of this. My friend's Lyr wasn't able to bring the magic to the CD900STs that my Little Dot I+ was. I think they really need to be paired with a slightly mid-forward amp to really get them to shine. Similarly, the K501s probably need a less mid-forward amp to really shine, so maybe I shouldn't make concrete comments on them until I've had a chance to use them on another amp!
It's OK I also liked the Sonys when I heard them but I never had the chance to compare them to k501 side by side. Maybe I am a little biased to the k501 but those I have heard so far have been very special to me. Maybe it is my musical preference, as I listen mainly to classical music and asoustic jazz. We all have those headphone models that are little "special" to us. Anyway, as I have stated in other threads, I would be happy to find a mint pair to purchase. Unfortunately they are scarce today.