I really like my HD650's, but I love my HD800's. With or without steroids, I can't say that I hear any real resemblance between them and the lean sounding K701's (which I don't care for at all). Rather, to my ears, the HD800's definitely carry on the Sennheiser house sound (accurate and textured timbre, full sometimes lush mids, and solid but not bloated bass), while adding a bit of treble energy (although not nearly as much as say DT880's), a much bigger and better sound stage and more speed and detail than the 650's due to the ring drivers.
I know there are a handful of highly prolific posters on HF who regularly rage about the 6khz "peak" 800's (which really only impacts overtones), but at the end of the day, I think they simply represent disproportionately vocal minority of listeners. I expect most HD650 fans do/will/would really enjoy the 800's. I continue to thoroughly enjoy both of these headphones, which are, in my opinion, the best dynamics at their respective price points.