I heard a modded pair of HD800s several months ago...was very impressed, even though they are kind of the polar opposite of my usual headphone/voicing tastes. I heard the Final D8000 in 2 different booths/rooms at Canjam (the room was much quieter). Not a very long exposure, to be sure, but extremely impressive. So not prolonged experience w/either HP. Still, I'll go out on a limb and say these headphones don't sound alike. For example, the D8000 has what I regard as spectacular bass--reaching very low, having what sounded like excellent pitch differentiation & impact. Plus it has that quiet-then-BIG planar quality that's hard to describe...smoothness that gets big in a hurry when the music calls for it. I couldn't evaluation soundstaging very well in either setup, but it sounded at least very good, also very open/transmissive of sound in & out (that's generally a good thing in open designs IMO). The HD800s had an entirely different presentation: all about space, and to a lesser degree, tonality. Bass was better than I'd been led to expect (that could be the mod in action), but those aren't heardphones I'd turn to for state of the art bass. I would turn to them for state-of-the-art repro of acoustic, minimally miked music, particularly symphonic/classical/chamber/choral. Very different sounds. I suspect you'll find they match well because they're dissimilar, but each capable of great refinement/sonic distinction.