Well, if anybody from Sennheiser is paying attention, I want a blend of the best of the HD600 and the HD650:
A richer, fuller HD600, but without the bassy colorations and dark ooze of the HD650.
In other words, a more neutral HD650.
((I suspect that Sennheiser can't do it. I suspect that the way that they produce the creamy smoothness of the HD650, compared to the HD600/580, is by adding that very darkness or bassy warmth. Remove it, and you have . . . the HD600.
So I suspect that Sennheiser's current high-end product stagnation and impotence is due to the fact that 1) they can't do any better, and 2) enough of us yahoos keep buying the HDs.
After all, what are the alternatives? Screechy, metallic-sounding Beyerdynamics and threadbare, overdetermined [in an obsessive-compulsive sense] AKGs? Overpriced-yet-still-crude Denons, made by a company that may soon disappear from the face of the earth, or, at the least, be transmogrified into a lowest-common-denominator boombox manufacturer?))