My impressions of the X2 are pretty much the same as Hans'. The X2 is much bassier and slopes down harder before the first treble blip, giving that V shape sound. I think the HD6xx are cleaner overall. Personally I give the comfort nod to the X2.

Out of the box for a casual listener, I think the X2 would be preferable. Extra points for not needing an amp, and it doesn't really scale much.

How would it fare with the Modi 2U/Vali combo?

I just tried doing the mods that Marv suggested on the first page. That is, removing the damping directly behind the driver and cutting a small hole in the foam in front of the driver. Highly recommended! This doesn't really improve the very lowest bass all that much, but it does bring up that hump centered around 100Hz a tad and may even shift it down a bit closer to 80Hz. They sound much more lively and impactful now in this regard. This also really helped with some of the odd grain I noticed in the treble before. I noticed before when adding damping inside the HD650 cups (in addition to the stuff directly behind the driver), it did something weird to the treble despite measurements looking good or staying about the same in the upper regions. I was actually using additional front damping before this to boost the bass and help reduce that grain I noticed, but removing the back damping foam stuck behind the driver works a LOT better and actually allows one to cut down on the front damping from the stock config. So, in a nutshell, try the mods on the front page.

One small thing I added to these mods, though, was putting some rug liner on top of the silver mesh surrounding the driver, under the existing foam. This is only cut to cover the mesh, so it's much smaller than the foam, and has an opening in the middle where the driver is exposed. Essentially, this boosts the bass a wee bit and helps smooth out some of the "wiggles" in the treble. It's a very slight change.

Attached a quick measurement of the left channel after these tweaks. My particular HD650 has better distortion in the bass than my HD600 did, but it's still not the best I've seen. Still could do a lot worse. My pads are also a bit worn. New pads will lower the bass a bit and bring up some of the treble.

Yeah, I took some brief measurements without the front foam. Bass dropped a lot. Treble got a bit weirder. Didn't even bother listening.

So, yes, try the mods Marv mentioned in the first post. I don't know what the diameter of the hole was...I just put the HD650 under some light and drew a circle over in marker where I could see the driver opening (very middle portion of the driver). Cut it out from there with everything disassembled, to prevent foam crap from getting in the driver opening.

If that's still not enough in terms of that slight, hard-to-explain grain, try the liner directly over the silver mesh and under the stock foam. Might help.