SET amplifier recommendation

I own 804S and when searching for amps I listened to them in the same room and setup with McIntosh MC252 (250W SS) vs MC275 (75W tubes), and preferred the 275.

Also, the 804S is not the same thing as the original Nautilus 805 stand mount speaker, which is what I assume the OP is using.  I have never met a tube amp that could provide speed or decent bass with the any of the original B&W Nautilus line.

Line Magnetic, KR and NAT. NAT makes some incredible sounding SET gear that used may fit your budget and will drive the snot out of those B&Ws.
Good luck!

I have an idea that might work, emphasis on the word might. This amp right here: https://diyaudiostore.com/collections/amp-camp-amp/products/amp-camp-amp-kit?variant=7072933085218 is an easy to build single ended transistor kit. It sells for $327, but if I recall shipping is expensive because the case comes from Europe somewhere. It is designed by Nelson Pass, and has a true single ended sound without the impedance and damping problems a triode tube would have, but it is still only 6 watts. Although I bet this amp would drive your speakers, it would not be ideal. I agree with the consensus that different speakers would be a requirement for true single ended nirvana. The trick is; it is a few hundred bucks rather than a few thousand and it would give you a taste of what the single ended craze is about. You would have to learn to solder if you don't know how already.

After I built my first S.E.T. amp (45 tube, 2 watts per channel), I spent 20 years looking for a true full range speaker for it. I wound up building a large, make that very large (even I wouldn't have it in my living room), Altec based horn speaker with a RAAL ribbon tweeter. The system is tri-amped. I use this amp right next to my S.E.T.s ,for driving the tweeters.

@jdl57 A speaker change to Coincident PREs (from Coincident Super Eclipse IIIs) caused me to have to change amps from Coincident Frankenstein 300bs.  Eventually I built my own First Watt F4s and find them excellent.  I was very surprised, because I had previously demoed a Pass Labs XA25 and did not end up keeping it (although it was excellent). 

The F4s are certainly not SETs, but surprisingly, I don't really miss my Franks at all. The F4s create a ton of space and have excellent tonality.  A touch of sweetness while also being incredibly transparent.  They don't distort at all and play better at the extremes with the PREs (94db and supposedly a flat 8ohms).  Plus, as you point out, DIY does save a lot of money.  I built two monoblocks and vertically bi-amp my speakers so each driver gets its own 25 watts.  Under $1800 total cost.  I sold my Franks and upgraded tubes for around $4000.

@cygnus_859 Also, I know DIY isn't for everyone, but if it does sound at all attractive, I would just ask the community over at:  https://www.diyaudio.com/forums/pass-labs/ which amp build would best fit your speakers.  A lot of very knowledgeable people over there (including Nelson Pass himself).  I kind of wonder whether they might recommend an amp with light feedback for your speakers...