RickB
Headphoneus Supremus
I want to update this: once the HD600 fully burned in, I found the difference between the 600 and 650 narrowed enough that I now slightly prefer the HD650 for its warmer tone. The HD600 is a little brighter, but I found the effect diminished as I put more hours on the phone. I also tried using an HD650 cable on the 600 and that made them harder to tell apart.
I'm happy to own both headphones and I will keep both.
Never mind. I did a back and forth between the two headphones and find that I prefer the HD600.