Well, there's no going back. I bought a pair of used HD650s for a friend and, while they were passing through, took the chance to spend the afternoon listening to them side by side with the HD800s in my system. The 800s simply pissed all over them.
Despite having lived very happily with both 6X0 series Senns in the past for extended periods -- they now seem uncomfortable, lethargic and muffled to me by comparison. Off you go!