I know the issue you speak of, but the D600 doesn't have this behaviour. I can turn my head side to side, and nod up and down and not only does the headphone not slide off, but it also keeps a seal.
Btw- the D600 is even more comfy than the HD650. Though I think my HD650 is starting to fit better since I've got it.
As far as price goes, neither the D600 nor the P7 are worth their MSRP or typical retail prices for head-fiers that are savvy enough to hunt for good prices or used gear. The P7 did strike me as a better Momentum (initial impression, would have to have them for a while to decide).