SET amplifier recommendation

I have tested a B&W 805 with an Ayon Mercury 300b Set amp (20W). Very bad match. 

You have gotten plenty of input, but I thought I would chime in because I have 805's and have hooked them to many different amps.  The sensitivity is definitely an issue.  I have had them hooked to even some higher power tube amps(not SET) like Cary, McIntosh, etc, and they just didn't sound good.  They definitely sound significantly better with more power.

As far as replacing them, if you really like the B&W sound, you won't be happy with vintage Klipsch, IMHO.  But getting new speakers is a whole different ball game :) Good Luck!

I'll add my two cents here, as I had the B&W 805 speakers (D2, not the current D3) a few years ago, and while I no longer have them I'm considering going to SET with my current speakers.  I moved from B&W to Wilson speakers - first the Sabrina and very recently with a used pair of Sasha (series 2, not the new DAW).  In addition to my solid state amp in my main setup (D'Agostino Classic Stereo - 300W at 8 Ohms, 600W at 4 and 1200 at 2) I have a Cary CAD300SEI integrated in my office setup.  The Cary is rated at 15W.  Enjoying what the 300B SET does for my office speakers (Omega Compact Alnico - single full range driver and 94.5 dB sensitivity), I had some friends over to compare it to my solid state driving the Sabrinas.  There was certainly something magical about the midrange, and I didn't really feel the volume or dynamics was lacking, but we agreed the solid state power amp had better control of the bass.

The Sabrina has a sensitivity of 87 dB and a pretty challenging impedance curve.  So it was an interesting experiment but not really a serious contender for replacement.  The Sasha's, however, are a different story.  They also have a difficult impedance curve, but with 92 dB sensitivity they're not as challenging.  Same comparison, but this time I'm thoroughly entranced by the SET amp driving these speakers.  There was plenty of volume and dynamics for me, and I didn't feel there was anything lacking in the bass compared to the solid state amp.

I should also mention that my listening environment (my living room - not a dedicated listening room) is pretty lively, with hardwood floors and, though softened with rugs and cloth upholstered sofa and chairs and drapes over windows, I've done no special sound treatments.  I know that when I've auditioned new gear at my local hifi dealer with the exact same components I have at home I use a much lower volume setting on my preamp than I do in the dealers highly treated room.  I also have to position the speakers fairly close to the front wall (less than two feet between the back of the cabinet and the wall).  The point is that the room is going to have an effect as well.

To me the bottom line is that I'd suggest you find amps that you can try in your setup at home.  There's really no other way to tell if it might work for you.  To add to the fun, if you could find other speakers to audition you might really have a fun few days.  (Auditioning gear is what I miss most about having my system mostly sorted out).

@cygnus_859 If you're curious how much power your speakers are using and have (or are willing to buy) a multimeter, you can use the technique at this link to actually measure how much power you'll need..

https://www.diyaudio.com/forums/multi-way/204857-test-voltage-power-speakers.html

And since some of the information is buried in the thread, here's a quick rundown:  all you do is set your volume at the max listening level you use, then play the provided -12db 120hz test tone while measuring voltage at the speaker terminals. Square that value and you’ve got the maximum watts needed. Plenty to read at the link...