I've never heard it with the HD800, but I would describe it as neutral and extremely transparent.
Here's a Head-Fi member (FlySweep) who thinks a lot of the HD800 with MG3 (using an impedance match that Jan Plummer optimized for the HD800): http://www.head-fi.org/t/649107/speaker-amps-for-headphones/1005#post_9794963
I totally understand how many SET owners have found the Millenia [MG3] to be one of the first solid state amps to satisfy them. The sound possesses a uniquely elegant warmth & extremely refined liquidity.. while simultaneously sounding unbelievably clean, speedy, clear, detailed, and boasting excellent bandwidth. I (personally) haven't heard anything quite like it, actually.. it's, quite simply, mesmerizing. How it manages to sound so accurate and transparent through all this.. I have no idea.
It's accurate.. but not remotely fatiguing, clinical, or harsh. The background is remarkably silent. I've never heard dynamic swings come across so smooth, well controlled, and effortless. The tonal purity is hypnotizing. I simply love linear, wonderfully extended, tight, punchy, fast bass & crisp, airy, open (and again, linear) treble.. but the Millenia's midrange, is nothing short of sublime.
This is, without a doubt, one of the finest amps I've heard the HD800 with.
And with the HD600 (using the same impedance match): http://www.head-fi.org/t/649107/speaker-amps-for-headphones/1140#post_9851367
One thing to keep in mind regarding the impedance matches that Jan Plummer builds. Aside from being matched for a specific headphone - gain as well as impedance (it's amazing that he takes the time to do this for his customers), the $100 impedance match boxes come in two flavors - one for use with balanced headphones and one for use with single-ended (TRS) cabling. Pick one or the other or both, but don't try to use an adapter to get from the balanced impedance match to a headphone with single-ended cabling, because... the MG3's speaker terminals do not have a shared ground.