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

Back in the late 80's I was evaluating a pair of Apogee ribbon panels, stunningly beautiful and wonderful sound. The dealer modified the 555 to drive 2 ohm loads, if I remember the costs was 1/2 the power but I could be wrong. Still using the Adcom today but I returned the speakers.

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.