I dont think the 650 would give you anything more of what you are looking for. they are not better then the 600, they are just different. The bass and treble are better on the 600 in my experience. I had the 650, and I traded for a 600. no ounce of regret.
You should sell both cans and get the HE-500. IMHO.
Nah, the HE-500 is regarded as really not that much better than the HE-400, with the bass being better on the 400. The thing is, the HD600 is a little too close to the HE-400. I want something liquid smooth. Many have said the Lyr+HD650 is a winning combo, and I know the HD650 is hard to drive properly. Like I said, I think I'll try to audition them first.