One area that the HD800 just can't match the HE6 is bass extension, force and speaker/subwoofer-like presentation. I find that unique to the HE6 when well driven. The LCD-2 sounded like an average headphone in comparison (and everything else I've heard).
In terms of morphing with changing amps/sources, the HD800s wins hands down. Yes, the HE6 becomes better with more power and quality, but it doesn't change its signature as radically as the HD800 from setup to setup. I still find the HD800 to be the better of the two in terms of imaging, soundstaging, speed and microdetail. It's not a night and day difference and I think the HE6 sounds more speaker-like and does some aspects of bass better as I mentioned before. Plus, it's more forgiving than the HD800, not scrutinizing and punishing recordings/sources as much. I also find the HE6 keep sounding better and better at higher volumes without getting fatiguing. This is another quality I've not found in any other headphone. (I'm normally a low-volume listener, but the HE6 just makes you want to turn the dial...)
I don't know which I'd choose if I could only own one. Probably the HD800, but comfort would play a role in that.
Good post Shah,
Of course the last sentence will always be about ones preference.