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

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. 

Thx Soix so much for your rich expertise. Unfortunately, in my country, only Krell, Mcintosh, Passlabs, Rogue Audio, Plinius are available. Would you mind share the sound difference for each of those.

@truongv0ky I ran B&W N803 with Pass XA250.5 and Audio Research LS-25 preamp. Great combo. The XA series would work well with the 805s.
I think the sound you’re looking for you’ll find with Pass XA-30.8. 
I had Rogue separates (ST-100 and RP-5) and while very good the Rogue stuff tends to lean slightly towards dry sound in my opinion. 
Also, your Chord Qutest will be a good match with Pass Labs gear.
The Qutest has great tone and it won’t be masked or veiled by the Pass amp.

Well, I’d rule out the Medusa for previously mentioned reasons, and I’d rule out McIntosh because the treble will likely be too reserved for your tastes.  I’m not familiar enough with Krell as they seem to have shifted back to their more classic sound, which is a good thing so worth exploring.  If looking at Pass I think the X150 would be the one to focus on as the XA series might be a little too soft in the treble for your tastes.  The one that intrigues me is the Plinius P10 that is considerably cheaper than the others but seems to exhibit sound characteristics that are close inline with what you’re looking for.  This review may be of interest to you…

https://www.whathifi.com/plinius/p10/review

Thats all I got for now, and I’d encourage you to read as many reviews as you can on these three amps to see if one seems better suited to what you’re looking for than the others. 

McIntosh pairs excellently with the B&W diamonds.  The newer Macs are not reserved or rolled off but smooth enough to give your speakers the ultimate in resolution with none of the excessive artificial brightness.  Heard this pairing many times and also Mac with even brighter speakers like Klipsch Heritage and they sounded like a winner every time.  Mac and Harbeth? Not so much but with the diamonds the system shines.