Using Adcom GFA-555s as monoblocks to power magnepan 3.3s in active biamp

I did much like you I had a pair of sim audio w3 moons and a bryston  10b active crossover. My room was 16x28x8. There was absolutely  no lack of power there. I really  doubt you are going to need more than you have running them as stereo  amps. 3.3 are power piggies but you should have lots. 

 the bass panels will still be 4ohm but the amp will see a 2ohm load when bridged.

I had Adcom separates in the 90's.  They are not a high quality product.  I actually GAVE them away.

I think the poInt is bridging disadvantages the amp’s ability to drive low Z loads. 
You have one. So why would you think it a good use of your gear?  The 555 was heralded as “high current” in its day (relative to its class peers, not Krell!), but bridging compromises that quality. I would try vertical bi-amping, however. 

Vertical biamping is an interesting idea - as I've always used different type amps it's always been horizontal.  But I have better amps than the Adcom as a stereo amp to use for the biamping (except the vertical as an experiment).  

Right now I do have a Krell amp as part of the biamp but it's on top.  I figure the better spot for the Nakamichi is on the bass.