I think there were two models of each, where the later model 650 had clearer highs or less of a veil. I believe both of the 650s that I tried and sold off were the earlier version.
I sold the first 600 that I had as well, which I think must have been a later model. The later model 600 has sharper highs that actually got to me. I still have my second 600, I think it must be an earlier model. It's the headphone that I have owned the longest and I still appreciate it.
That's how I think it went with the early/current 600/650 sound signatures but I could be mistaken. I do know that it's relatively easy to visually identify which is which with both headphones but I don't remember the details.