My HD600's came in today! I only got about an hour with them, but wow. 'Natural' really is the best word that comes to mind to describe how they strike me. Nothing seems boosted or cut; every instrument appears to come through as it would sound in the room with you. I can see how those in search of a more 'exciting' or 'enhanced' sound might be turned off by this, but for me they are perfect. I am currently running them through a HeadPod 4 from my MBox 3 Pro. I am sure there are better setups out there for the HD600, and I hope to experiment with some different amps down the road, but for now everything is coming through clean and believable, so that (as others have pointed out) I might just now be able to listen to the music rather than headphones.
Thanks to everyone on this thread who helped to push me over the edge!
I can't wait to put them on again tomorrow