Headphone amp?

You can occasionally find HD800s headphones used, but expect to pay ~$1200 for a complete boxed set in excellent condition.  I waited several months for the pair I recently purchased.

I run mine with a Quicksilver tube headphone amp and Moon Audio Black Dragon cables - a setup which, by audiophile standards, doesn't break the bank. For my listening tastes (mostly female vocals), I rarely take the volume knob past 10 o'clock. (If $$ were less of an issue, I would have looked closely at the Pass Labs HPA-1).

The 800s cans sound wonderful, though I find the HD600 series to be slightly more comfortable.  Still, it's a worthwhile trade-off.

As always, YMMV.

Good luck in your quest!

I use Moon Audio Black Dragon headphone cables, too - I've got them for all 3 of my headphone sets. The Quicksilver tube headphone amp is some serious value for money at under $1000. 

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.

I stopped looking after I got my Simaudio 430HA. I also like the Sony TA-ZH1ES if you’re looking for something more affordable. I have HD800 (not S) and they sound great with either amp.

I use a Woo WA22 tube amp with my HD800s, and I think it sounds great.

The signal is run balanced with XLR connections from a Sony XA5400 SACD player. 

With the ability to roll different type of tubes you can tailor the sound.