Need your Opinion

Both amps are likely going to have enough power to handle the dynamics and soundstage you want from the speakers. The Krells are strong! The Full Power Ballance is going to sound better however, in most all respects. If you need an upgrade in overall sound, the FPB is going to improve things.
As for Krell for your centers. I see no reason to compromise or change things up so much. I'd stick with another krell at your level. However, you could substite very close sounding amps to your Krell KAV, should you stay with that amp. The ATI and possibly McCormmack multi channel amps sound a lot like the Krell in terms of tonal balance and extension. Might make more sense monetarily anyway to add a 5 channel ATI to the center and rear speakers, and keep your krell up front/left/right.
Hope this helps

FPB200 is much better amp,, you may hear better/tighter bass and more details,,in comparsion,kav is grainy. regards

I would try to stay in the Krell line if you can swing it,if not you should stay as close as you can to get a nice clear transfer of sound from one speaker to the others. If you have to use another brand of amp then do as foreverhifi says and stay with something close (McCormack,Aragon) would work I would think. I have a Krell Ksa-250 for my fronts and a Krell Kav-500 for center and rears and the blend from speaker to speaker is great. I do use the same brand of speaker also as you would be doing for fronts and rears.