Replacing my Krell FPB 600 amp with a tube amp

I went up the Krell line of amplifiers in the 1990s, and ended with a FPB 600, which deserves high praise, but has certain - let me say - conditions. In my system, it sounded much better when it 1) got a dedicated power line, 2) a solid slab of stone below, 3) a good tube preamp rather than a mid-line Krell or other solid state (the manual warns of possible problems with a tube preamp, a jumper can be used, but I noticed much better sound without it, and never had problems), 4) using balanced inputs, and 5) matching speakers. The ones I had (Dynaudio Consequence) were a bit too much even for the 600. So the sound went from good to marvellous only on a quite high volume level (the amp shifting into class A operation). Even this power beast could not really unsettle the rather Germanic Dynaudios and make them sing and dance. Experimenting a little with speakers, I marvelled at how good the amp could sound driving an easier load. Since it was not possible to add another 600 amp to my setup, I ended selling both the amp and the Dynaudios, and bought Atma-sphere MA-1s instead.

I could say "I have never looked back", which is mostly true, but having lived with the Krell for ten years I have a lot of respect for it, it is a classic in some ways, and I have often wondered if there was even more potential to it than I managed to exploit in my setup. It is a bit of a paradox, the FPB 600 was sort of a "solve-all" when it came, it should not matter what speakers you gave it, it ate them all, and so on - but my experience was different, this amp is so good that everything you do - in terms of speaker matching, and the points mentioned above - is well worth it. Just before I sold it I had the Krell drive my small Aurum Cantus 3 speakers to great heights. 

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. 

This was some years ago, but I remember thinking: connecting a 1k dollar set of speakers (the Aurums) to a 12k amp cannot be any good. So I was surprised.

I often wondered if I would prefer a different bias system on the Krell - an option to make it go into class A operation on a lower volume level, or more user-controlled. Maybe it came in a later version (mine was a 1997 first version).

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. 

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).

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.