HD600 are a bit more neutral than HD650 (less veiled higher mids).
I don't know if they're really what you're looking for, but as long as you amp them right they'll sound great. They were the headphones I preferred amongst all I tested in a music store. (Naturally, I tested them against the HD650 too). IMO, the AKG's mentionned earlier in the thread weren't as good sounding, nor detailed. I reckon they had good mids but if mids were the only factor (assuming you don't mind open headphones), I'd go with an AKG K701. If you need balance all over the spectrum, you could go for HD600.