So, HD800 > K1000.
Edit:
In the words of Chuck Palahniuk (the writer of Fight Club), the K1000 / First Watt F3 combination has, for the most part, made me go a "big rubbery one."
The headphone is pretty cool I must admit. And the sound isn't half-bad, but next to the HD800, it falls short in the technical areas of: extension, resolution, and soundstage. Yes, even soundstage. Even with it's stretched out horizontal stage and binaural trickery, the K1Ks fall short of the HD800's depth & presentation. Ultimately, the Senn is the more convincing one of the two.
The F3 clone actually needs to go to 2pm to drive the K1K (ridiculous), and it's the oddest solid state amp I've ever heard. It has slightly polite bass without too much kick/attack and a "liquid" midrange. The presentation is amazingly laid back. I think this might just be the "Pass" house sound and it's pretty damn good. Unfortunate that it doesn't have more headroom for the K1000. There are a lot of reports out there of this amp sounding like a good 45 SET tube amp, and they're probably correct.
It does kick the LCD-2's ass, and I'm finally not underpowering those. The LCD-2s sound better than they ever have in my system. The separation is finally not falling apart in busier passages, and the sound is so smooth w.o. any signs of strain. But again, even with those LCD-3 pads (which help a lot) the LCD-2 is close, but not quite at the level of the HD800.
And to be honest, my HD800 rig isn't even that great. I know a better amp would provide more slam and a more cohesive presentation (in terms of tone thickness...ie. non-thin) - thinking of Nikongod's Silver Ghost amp here.