Very similar, but the K7XX has a larger sound stage, and might have a bit too much treble in comparison to the HD6XX, but some like that. The HD650 has a noticeable mid-bass hump. Some people like this, others don't. What it does is give the bass a bit more impact. However, I'd say that the K7XX's bass, while being more linear, extends a bit deeper into the sub-bass.
Honestly depends a lot on what amp/dac you use, currently using K7XX and it sounds fine, but I would probably trade it for the HD6XX if I could and I also only listen to EDM.