Replacing my Krell FPB 600 amp with a tube amp

Note: my version of the amp does not make it clear when it shifts into class A; the nearest I could get was to feel the heat on the top plate. As soon as I turned the volume up with the Dynaudio speakers, the top plate turned hot, playing loud, very hot. So it may be that the amp was already working in class A, no problem there, but it was striving with my over-controlled speakers, which would have liked double power - 600 watts per channel (I never got a chance to try this).

I heard an interesting transformation of B&W 801 Matrix S3 speakers driven by Pass Aleph 1.2 mono blocks (200W/Ch).  They became smooth and rich even in the treble, which before had always had a bit of harshness to them.  So budget permitting if it were me, I would consider a Pass XA amp to replace the Krells. Rowland amps are also "tube-like" although rare on the used market.  I am also not sure that your speakers really need 600W to begin with.  One caveat- I agree with Mr. Holter that the Krell FPB's are very good amps- I had one myself and it was a joy to use- reliable, quiet, unflappable, easy on the eyes, etc.  

Maybe I am wrong, no expert here, but my impression is that the B&W Matrix 800s are quite a difficult load - or at least, that owners have long been looking for the best ways to get an optimal amp match...On paper, the Krell FPB 600 should be able to drive the Dynaudio Consequence with no problem. In reality that was not the case. It was good, yes, but not optimal. The Consequence speakers were developed using a 1200 watts amp.
Then, of course, there is the question "what kinds of watts". It is hard to say exactly, but I would guess, in terms of loudness, the 140 tube watts on the MA-1 translates to 450 or so solid state watts, it is not as loud as the Krell system, but not far behind. I miss some of the bass action and authority, but not much, due to the fact that I learned from the failed Krell - Dynaudio affair in my system. I bought a speaker system PRECISELY tuned to the amps. In some respects it is simpler than the Dynaudio speakers plus Velodyne sub I had, but the matching makes it stand out. So I think, the place to start is not this amp or that one - but what kind of sound, what kind of speaker match. If in doubt, go for more easily driven speakers. And consider OTL and good tube sound. That was my way, based on listening tests. 

@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