Appreciate the insights. I should have made it clear, I already own the X1's and I absolutely love them, I just want to get some Senn's and have read that there are similarities in sound between the X1's & 650's, so I think I'm leaning towards the 600's at this point, to experience something different.
I just wrote a thread comparing the two cans, actually. I have both. The HD600s are incredibly.