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

And, why does a single GFA-555 not do it for you, sounds like a room issue or some kind of a synergy mismatch. A 555 is no slouch in stereo mode, 331.9W into 4 ohms. If that’s not enough power, something else going on there

There's no problem to solve.  I had a single Classe CA-200 powering the bass panels and it sounded wonderful.  I moveed the Classe to power some newly acquired KEF R11s and temporarily put in a Nakamichi Pa-7 powering the bass panels, and it still sounds pretty good.

But since I had the open question of what is best in place of the Classe amp, and I have two GFA-555s, I thought why not try and see if they can well power the magenpan bass panels as monoblocks. Intuitively it seems two monoblocks of that power should do better than a single Nakamichi, but not if it's going to damage anything. 

The 555 will power Magies just fine but I would not bridge them. 

Thanks.  Seems to be the consensus that it will "work" but not sound great, with some chance of actual trouble. I may stick with the Nak there for a while - it's not bridged, and it's beefy.

My 16 year old came upstairs and asked me to turn down the volume!

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.