I owned the HD650 and they sounded veiled to me. I like a somewhat bright sound and that is not what the HD650 is. You might want to research them before you put down your money. Many people love them, and many, like me, hear them as veiled. You'll have to decide for yourself.
I've owned the HD650 and sold them quickly. They sounded very veiled to me. From what I've read, if you really like the HD600 you would not like the HD650. I had the HD650 a year ago and I have the HD598 now. Auditory memory is not real good but I believe I like the HD598 better than the HD650. Of course you can cover yourself by buying the HD650 and selling the one that you like the least. Its not likely to cost you a lot.