Do you by chance have a nice speaker amplifier in your home? If you do, I would have some speaker tap cables made and simply use the speaker terminal output to drive the 800S. I do this with my NAD M3 which is a 180 watt amp that had no headphone input. I have driven quite a few headphones directly from my M3 using the speaker tap cable such as: ZMF Vibro X. MDR Z7, TH 900, TH 600, HD 800S, HD 600, HE 560, HE 400i, ONKYO A800, LCD 2F and a few others.
My point in providing the list of headphones was to demonstrate that there is no headphone that can't work with a speaker tap cable. I sold my LCD2F a few weeks and the buyer couldn't believe how amazing the LCD2F was from the speaker taps. This man owns pretty amazing gear Accuphase and Harbeth so he knows his audio as he has spent years getting his sytems right. We were discussing the possible headphone amp he might get when I asked him do you really think any headphone amp out there could possibly hold a candle to his Accuphase amp? Of course not, not even the Blue Hawaii. Speaker amps have plenty of power due to big beefy power supplies.
I'll add to this suggestion. So one day I'm in a coffee shop and I overhear two men talking, they are obviously both audio engineers. One was telling the other about his time in China. I introduce myself and sit down with them. The one returning from China had set-up and run a production facility for Bose, so safe to assume he knows a thing or two about audio engineering. The other man was a designer of power supplies and he had designed many headphone amps including some for brands commonly discussed here. When we got talking about the heart of any really good amplifier the biggest factor they agreed is the quality of the power supply. So again, if you have a nice sounding solid state speaker amp why spend a $1000 on another box, simply get speaker tap cables made and take advantage of what you already have (assuming you do have one). Now I'm not talking about the nasty cheap stuff, but any decently designed amp should do. As long as you like the sound signature of the amp, that is what you can expect from the headphone if you can predict how they will sound together.
I asked
@StanD
about whether or not my speaker amp actually is working too hard given the headphones are much higher resistive loads than my speakers and he explained to me that this actually is easier on the amp not harder. So as long as you have a volume dial your golden. I'm shocked more people don't use speaker tap cables. I had mine made by Trevor at Norne Audio and he did a fantastic job. I had him make it in two sections. One section is short and stays connected to the amp all the time with a 4 pin XLR termination. Easy to tuck out of sight when not needed. The longer section of course that goes with the headphones (detachable and I have a few adapters for most style of headphones) simply has the matching XLR connector.
I would love to buy a headphone amp, but there isn't one made that I could afford that would have any chance of being more capable than the NAD M3 so sadly I just don't need one. I did add an OTL amp for a different flavor mind you, but I don't need a SS head amp anymore. Just some thoughts to consider.