Replacing my Krell FPB 600 amp with a tube amp

Almarg - I was not trying to say that the B&Ws are a good match with the Atma amps - most probably not - the OP would have to change speakers in that case. I agree, the zero autoformers is a useful add-on, but again, only if you don’t have an optimal amp-speaker match, which is a far better solution. If the B&W specs say they want 150 to 800 watts (solid state), maybe they would shine with 800 watts or similar powerful amplification - I have no idea, just a guess, based on my Dynaudio experience.

So here is my bad take, overall. 

The Krell's that I remember in the day of the B&W Nautilus were cold and analytical as hell. Having heard the top of the line, 5 amplifier setup with Krell crossover and B&W Nautilus speakers I was never wowed, but if you were that's cool. 

If you want to keep that, you want old ARC. If you want something musical and juicy you want Conrad Johnson. 

I also remember the Nautilus, a bit cold and analytical yes - I preferred the Dynaudios for that reason. But I felt it was the speakers mainly not the amp. 

@tattooedtrackman - It appears that your ARC Ref 6 preamp has two variable outputs and one fixed line out for use with a tape recorder.
You pay your $14,000 but you get what you pay for!

I think @rcprince may be right. I have gotten very good results from VTL MB450III mated to my ARC Ref 40 to drive my Q3s. The variable damping factor on the VTLs allows you to optimize for the speakers you are using and address the issues @almarg raises. The VTLs are also very robust with super reliable automated biasing. Even a full on tube failure is handled with aplomb. If you do go the VTL route however you are advised to buy from a VTL dealer as they can be sketchy on support if you don’t have that relationship. Finally make sure to connect the pre and power amp with balanced interconnects.