The D7000 runs fine just off the Mixamp. The benefits of extra amping won't be truly noticeable when you're too busy gaming with them. It's not like trying to run a 600ohm off a soundcard.
I don't notice clipping on my PC360. but definitely got clipping with my DT990 (though that was when running it straight off the Mixamp), weird.
Ah man I don't know what's up, just playing dead space 2 again, this time with dt 990 running through e9 to the mixamp and had major clipping on one particular section where lots of enemies were coming at me and the sound totally cut out for about a second and was clipping a bit for about 3-5 seconds after the cutout