Sorry, neither the crack or s.e.x is done yet, so I can't tell you how they differ or how the 600s sound.
I've only used them both with a small portable amp, which I no longer own, and to my ears the 600s are more open and transparent without the overpowering bass of the 650s. I like them both but so far prefer the 600s. That said, you really should do your own comparison as our listening tastes may be very different.
-- Jim