Richard,
you obviously like the Krells my suggestion would be give a MONO pair a try, trust me you will love it. You will hear the difference right away, the sound-stage will open up more, a lot more depth and height too. I would just try the modrange or high end FPB MONO's.
FPB250M, FPB250MC, FPB350MCX = Same amp, low series, sounds ok...
FPB350M, FPB350MC, FPB450MCX = Same Amp, mid series, sounds Excellent, Love these amps!
FPB650M, FPB650MC, FPB750MCX = Same amps, top of the range, Sounds even better!
I did just order a set of FPB 750MCX's but honestly I really love the FPB350M series they sound fantastic. I was very surprised when I first played them and removed my FPB stereo amp, its a big difference, you wont regret it.
I have no idea what that gentleman is chatting about with the integrated and loudness button? I don't think an FBI has a loudness button? To me, every Krell I have heard plays just as well at low levels as it does at high levels, very little difference, these are high bandwidth, constant voltage source amps. I think speakers make a bigger difference on that subject?
Let us know what you decide?
Rob