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. 

Don't forget when you take part of the crossover out  the passive box and replace that part with the active cross over the speaker impedance  is likely  different  than what the manufacturer  has published.  When I had my 3.3 actively  blamed I had the horizontal  that way I was able to ture the base separately  form the mid and top end. By using different  powercords and interconnect s. 

I find it perplexing  what you really are wanting. On paper your amps are bigger than my w3 amps the moons were known to be able to swing large current.  But that being said Maggie's need that and not only was there never a need at any volume  the need of more power there was the sense of ease  that comes with a hi-fi  with big power. You don't need a power number to accomplish  great sound in fact the numbers game is a mid fi low fi game. In horizontal  configuration  you have separation before  the amps so that each amp only receives  the signal it is going to drive.. I should say that the room I was I n at the time wasn't  huge but not small either. Around 3200 cubic feet.