I did get some time to listen to the "over ear" Momentums. And if you can get over them touching your ears, which I find I don't notice after the first song, or until I take them off, the Momentums do have a similar sonic presentation. Sure they aren't in the same league as the 650s, but for a portable over ear, driven by smart phones at half the price... They are very enjoyable.
I'll try to listen to both more and provide more feedback.
The general things that I noticed were the sound stage was smaller and there was a lot less detail. But I thought, at least for the brief listens, that tonality wise they were certainly in the same camp.