Amps for B&W 805 Diamond: Rogue Audio or Krell, Pass labs, ML, Accuphase

The Medusa is a great amp with outstanding detail and 3D imaging/soundstage but I wouldn’t recommend it for the 805s unless you’re willing to do some tube rolling so of the two I’d side with the Krell.  With Pass I’d go with their XA series as it’ll likely play nicer with the diamond tweeter.  The Plinius SA103 is also a great choice, and it has a hi/low bias switch so despite being fully Class A you can leave it powered on or use it for less critical listening purposes without the amp running so hot.  Last recommendation would be the Clayton Audio S100, and if I had the 805s that would be my first choice.  Hope this helps and best of luck. 

Thx soix for your advice a lot. May I ask in detail why Medusa might not suitable for B&W. Rogue Audio is very good for bass so I suppose it is ok with B&W, isn’t it?

The Medusa, while being an excellent amp, definitely leans to the more detailed/revealing end of the sonic spectrum (at least with the stock tubes) and I’d be concerned that this combined with the 805’s likewise very detailed diamond tweeter might make the sound come off as a bit overly bright sounding and potentially fatiguing on extended listening. If you can share what sound characteristics are most important to you and what improvements you’re looking for it would be very helpful in making recommendations better suited to your specific tastes. BTW, what amp are you using now and what’s the rest of the equipment in your system (including interconnects, speaker cables, power cords/conditioning) and what are your listening room dimensions?  The more info you give here the better the recommendations here will be. 

My type of music are quite variety from slow music like pop ballad, country to soft rock like The Eagles. 

My most important sound characteristics are deep, soft, neat bass. Also a thick, warm mid-low and sparkling treble.

My systems include Dac Chord QuTest, NAD M3 amplifier, 805 Diamond, Sub Rel S510. Speaker cable is Ansuz X

My room is ~ 20m2 but it's a living room so sometimes I must open small volume but I still need the detail with that kind of volume.

Hope to have your reply soon, & thx for your very generous advices

My most important sound characteristics are deep, soft, neat bass. Also a thick, warm mid-low and sparkling treble.

The only thing the Medusa will do well for you is the sparkling treble as its mids are not thick/meaty (although not thin) and the bass is about as tight and quick as any amp I’ve had in my system and nowhere near soft sounding. 
 

I don’t know if you’d have access to this used and fully-upgraded McCormack amp wherever you’re located, but given what you’re looking for I’d be all over this incredible amp because I think it absolutely nails the sound profile you’re looking for and is being offered at a very fair price.   I have the same amp at SMcAudio getting very similar upgrades now and can’t wait to get it back because with Steve’s upgrades these amps are thought to be at a near SOTA level. 
https://www.usaudiomart.com/details/649875150-smc-audio-modified-mccormack-dna-5-ultra-mod-with-gravity-base/

Or there are the other amps I mentioned before that I think would also be excellent except maybe not as much sparkling treble.  Other amps you might look into are Belles and Parasound.  Hope this helps.