Well, I took the plunge, and you are correct in every way. These are a significant move from the ATH-M50, and I am very pleased with them. The signature and SQ are on par with my HD-650's, and I do not feel like I am giving anything up when I want to be isolated from external "noise".
I heard the 650's again last weekend at the SF Bay area meet - I agree. If you like one or the other's signature and SQ, then they are a very complimentary open/closed pair. I'll eventually add the 650's. I love them every time I hear them.