Replacing my Krell FPB 600 amp with a tube amp

@tattooedtrackman "why would i need a subwoofer? "
You may not need one. You could try the ARC 250s first and add a subwoofer as needed. I got the impression that you are a tattooedBASSman and tubes just aren't going to give you the "power and bass slam of the Krell FPB 600". I'm driving my B&W 804 D3s (two 6 inch woofers each) with 300 WPC and I'm using a 12 inch powered subwoofer as well.
With cars it's cubic inches. With Bass it's driver size.

The impedance curve for the Matrix 800 that is shown in Figure 1 of Stereophile’s measurements indicates an impedance in the vicinity of 4 ohms in much of the bass region, as I mentioned previously, and more than 10 ohms at almost all frequencies above 1 kHz. That strongly suggests that the speaker is intended for use with amplifiers having low output impedances, and correspondingly having relatively high damping factors. Which disqualifies many tube amps as being suitable matches, regardless of how powerful they are. B&W’s recommended range of amplifier power, 150 to 800 watts, further reinforces a conclusion that the speakers were designed with solid state amplification in mind. (Solid state amps almost always have very low output impedances and relatively high damping factors, those two terms being inversely proportional).

The likely result of driving speakers having this kind of impedance characteristic with an amp having a highish output impedance/low damping factor would be weak bass, precisely what the OP wants to avoid.

While as many here realize I have been a great supporter of Ralph/Atmasphere and his products and his many contributions to this forum, the 2.3 ohm output impedance of the Atma-Sphere MA-1 makes them a non-starter for use with this particular speaker, IMO. Certainly without using the amps in conjunction with a pair of Zero autoformers, and very possibly even with them. And I’m doubtful about the more powerful MA-2 as well, with its 1.75 ohm output impedance.

I’m pretty certain that the CAT JL2 that was suggested, which I see has a specified output impedance of 0.7 ohms, would be a suitable match. As would some or all of the larger ARC amps that have been referred to. Not sure about VTL or CJ.

Regards,
-- Al

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.