I disagree with Raul on this one. Yes synergy is very important, which is why I suggest you stick with Mark Levinson as your main amps and also as they're the preferred amps to match with the terrific 32 preamp.
As for the low end, within rather broad limits, you could use just about any SS amp that has enough power and damping: just about anything with 200W/ch @ 8 ohms, i.e. Bryston, ML, Krell, Parasound, etc, etc, it really doesn't matter.
If you believe Raul is correct about all the amps (in a biamped system) having similar characteristics then the logical conclusion would be to go with all ML amps. In other words, sell the FPB 600 and get a ML for the subs too, but I really think that's unnecessary. The most important synergy is between the preamp and main amps, not between the main amps and the subwoofer amps. As proof, look at all the biamper-s who use tubes on top and ss on the bottom -- synergy? I don't think so.
I didn't know Raul was such a big Krell fan, ha ha ;--)
.