I received my HD600 a couple of weeks ago, and am growing to love the sound signature. This is my first mid-fi set, as I've been using AD700's and SRH440's as my main cans for the last couple of years. Recently the addition of a C421 and an ODAC to the collection prompted me to finally invest into a higher end set. The C421 was driving them admirably, but last week, I found a good deal on a custom O2 and have been driving the HD600's with it the last few days.
The sound quality at lower volumes was what most impressed me, as it was much clearer sounding than the C421. Even going up the volume chain, the separation seems to better with O2. I'm extremely happy with my current setup right now, it's given a lot of life back to listening to my music collection lately :D