Thanks. OK, now that I have both, I'm kind of amazed at how much I like the HD600 also. It has a more direct sound, with the vocals and central images just coming at me with great detail, force, and a wealth of texture, and even some shout, but all in a non-abusive way. It sort of leans to the Grado flavor that I also like.
It's very dramatic and compelling and fun, whereas the 650 is more ambient, with more hall-like overtones, where I am more aware of the atmosphere, reverb, and things in the very low sub-bass and the uppermost highs.
With the 650, I often keep turning up the volume to hear more presence, but the 600 is is just right there and present already, so I find that I listen at (slightly) lower levels and don't feel the need to pump up the volume.
Which of the 600 and 650 I like better will depend on my mood. I see what is meant by the 650 being more refined.
The 650 is more speakerlike, and the 600 is more in the traditional headphone mold, where it just grabs me and rivets my attention.
So these are different sounding headphones, more than I expected, and I'm keeping both.
I think I see the evolution of Sennheiser sound, where the 650 sounds like an effort to get a larger, room-filling sound with less central image emphasis than the original 600, and later the 800 went much further in that direction, with super-soundstaging and a kind of distant imaging approach (which I find to be not engaging enough and is why I sold my HD800 an reverted to my 650 and now both the 600 and 650).
It's amazing what the 6xx series can do and is a big favorite of mine among headphones of any cost. The 600 especially is extremely compelling. Both it and the 650 are absolutely great and I see why they are classics, the dean of high end headphones IMHO.