I drive Sennheiser HD800s (as well as some planars) with Simaudio MOON 430HA. The Simaudio 430HA was chosen after I tried a few others. Highly recommend for the great sound and the ability to drive demanding headphones.
The sound is detailed but not analytical, closer to the tube sound than many other non-tube amps, and without the hassles of dealing with tubes. The amp is powerful enough to drive HD800s, and HIFIMan HE6SE planars via balanced output even if you want to play with equalizer. The amp can also come with an optional DAC module, which is decent, but a bit lacks soundstage in compare to a top-end dedicated DACs. Overall, given the difference in price with/without the DAC is not that big, may worth it if one does not plan go for an expensive dedicated DAC. All the above is purely my opinion, I am not a dealer, just the end user.
P.S.
After i’ve got HD800s i was trying to compare Simaudio to some cheaper alternatives - including highly rated THX 789 - the contest was over after 5 min of listening to THX 789, I returned it.