Right now I use the HD600s pretty much all the time in preference to the HD650s. I'm going to hold on to both until I get the GS-1 and see how they each like that.
This probably opens me up for an ass-kicking, but I also feel the Enigma Apex cable makes a big difference with the 600s, particularly in terms of bass reproduction. Sort of takes them in the direction of the 650s, while retaining the greater urgency/immediacy the 600s have stock. Forgive my inadequate powers of description, please.