In my experience I find the HD800 easier to drive than the HD650 - which I also own.
All it takes to drive the HD800 is an amp that can swing enough voltage - it doesn't need very much current. So pretty much all stationary headphone amps are fair game.
The HE-6 I've never owned, or even tried, but from what I gather that one requires A LOT of current - much like speakers. Which is also why it seems that people who favor this headphone drive it from speaker amps.
I don't think a 50wpc channel amp would match well with the HD800 - well, some 50wpc amps might but it wouldn't be something to be expected.
HD800 is both lower impedance and higher sensitivity than HD650, so it's significantly easier to drive (50ohm difference....so basically there's an entire HE-6 difference in impedance between the two, with greater sensitivity than either.)
I think there's a bit of a myth around the HE-6 that it requires big power but doesn't care about quality so much, while HD800 cares about quality but not power very much. HD800 may be the king of absolute detail this side of stats, but HE-6 is very close in terms of its resolution and detail. HE-6 isn't much less dependent on quality of amplification than HD800 is. It needs the power in addition to the quality. So in that sense HD800 is the more forgiving of amps, though it just so happens that scales of economies make good quality high power speaker amps cheaper than a lot of high quality headphone dedicated amps. HE-6 may handle lesser quality amps with a lot of power more cleanly than HD800 handles a lesser quality of amp, but it rewards high quality amps just as well.
And on the flip side of the myth I was impressed with just how well HD800 handled lesser quality amps. But the reward for high quality is no lesser with the HE-6 despite the huge power demands. Therefore an amp that is good enough in performance to extract the best of the HE-6 is also of excellent performance to extract the best of HD800, provided one can tame the power output and make it suitably save. 300ohm goes a long way to doing that, though I'd still feel safer with even more extra resistance than the HE-Adapter provides due to the high sensitivity of HD800. The ideal, I suppose would be a preamp that was chosen for HE-6 that also has a direct headphone output tapped off the main amp with appropriate resistance added.
I just wanted to be able to see how it sounded with the amp but it is bought with the HE6 as it's pairing. I just wasn't sure if it was safe to use with the HD800 or not. I'm always on the lookout to see if my rig can get better at a reasonable cost. This gives me some opinions on it and I'll just keep the HE6 connected to the amp. I do have a good combination with the HD800 but the HD800 sounds better on the same rig than the HD800.
I'd love to try my 7025 with HD800. I suspect it would be a superb sounding combination. I'm just too nervous to do it, even though the numbers show it should be doable. That eliminates the expected results...it's the unanticpated ones that worry me
FWIW, Sphinx ran the HD800 off the FW clone....but that was only a 10wpc amp. 50wpc is a bit more of an issue depending on the gain factor, and my 140wpc amp would be downright scary...though it's very low gain and thus could in some ways be safer than a very high gain 50wpc amp like my PM6004.