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

Adcom GFA 555 and a Nak PA-7...that's a blast from the past !

They are fun amps.  I do have a number of more modern, beefier amps for the Maggies but I've been doing some swapping as I also added some speakers I wanted to hear with the better/newer amps.  

I think especially the Nak is a good value - bought it for like $250.  Basically a buck a watt for the current the thing will put out.  Also helps to heat the room in the winter!

I am using two Bryston 3B amps to biamp similar to as you propose but horizontal biamp. I use a Bryston 10B crossover. Per the directions in the Magnepan manual (I have IIIa Maggies), i do not use the crossover boxes and changed the jumpers on the back of the speakers. I can provide you with my crossover setting if you like. I use one amp for the bass and one for the midrange/high.

i do not use the crossover boxes and changed the jumpers on the back of the speakers. 

This is what I do as well. Magnepans are the best for biamping because of the external crossovers. I have a Marchand crossover - or to be precise a custom built one using a Marchand board.