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

((((Wow, thanks.  So, another question: is there any harm in running 1 channel of each Adcom?)))

that's the ok way if your going to do it.

While the Adcom is at its best controlling woofers like some of the infinity speakers

In my experience i cant thing of a worse combination.

These amps are way over damped for any of the Magnepans.

Try any vintage Bryston and you will quickly see what works.

 listen to The Planets, Gustav Holst 4th movement with the opening Acoustic Basses you will appreciate the proper dampening once you experience the difference.

 JohnnyR Magnepan Dealer

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. 

Adcom GFA 555 and a Nak PA-7...that's a blast from the past ! We were selling both of those brands w-a-y back in the early 1990's when I was running the Harvey Electronics chain of audio stores in the NYC/NJ area ! We sold a ton of the Adcom, fewer of the Nak as it was pretty expensive and though built well some people felt it was a little lean sounding...I think Nelson Pass may have had a hand in the design of that but not certain...

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.