I have both the HD650 and the RS-1, and I find that they complement each other beautifully. However, I haven't heard any of the other highly regarded cans that have been mentioned in this thread and elsewhere (e.g., W5000, etc.). I first had the RS-1's, and then bought the HD650's to complement them. Like the OP, I also have a SP Extreme (Platinum), which is wonderfully flexible and can be tubed to synergize oh so well with both of these very different cans. The RS-1's need lots of current to show their full potential, and the 6080 output tubes have power in droves, and (if your Extreme has been modded to handle them safely), the 6528's are phenomenally powerful. The HD650 is the polar opposite--doesn't require as much current, but needs lots of voltage gain, which 5998's or 2399's seem to excel at. As for gain tubes, there are so many to choose from with all the adapters that Mikhail offers. My current fav is the TS 2C51, which is very versatile and matches well IMO with both the Grados and the Senn's (can't say that about many tubes). Either way, the point is that, with a properly tubed Extreme, you can really make the RS-1's and the 650's sing.
The RS-1's really like tubes in the signal path IMO. I find that, with the right tubes, their slight "edge" gets ever-so-slightly mellowed, and they deliver an absolutely delightful, authentic, believable, warm, engaging sound. The RS-1's are not a "draw you in" kind of can (like the HD650's are) IMO; instead, it just puts it right there in your face and offers no apologies, nor does it need to. You either love it or you don't. But it's oh-so-easy to love with the right amp (one that has the right tubes in it IMO).
I love being able to go back and forth between the RS-1's and HD650's. They have the ability to project completely different presentations, yet both be completely enjoyable and fulfilling. It just depends on what kind of mood I'm in, I guess, when I decide which ones to reach for.
Only problem with the RS-1's IMO is that they get terribly uncomfortable after a couple hours use. The HD650's, on the other hand, I can listen to for hours on end with no discomfort whatsoever.
Good luck with your decision. Oh, I echo what a few others have said: don't sell your 650's no matter what you do.