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

 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. 

Vertical biamping is an interesting idea - as I've always used different type amps it's always been horizontal.  But I have better amps than the Adcom as a stereo amp to use for the biamping (except the vertical as an experiment).  

Right now I do have a Krell amp as part of the biamp but it's on top.  I figure the better spot for the Nakamichi is on the bass. 

What sound levels are you hoping to achieve? At low to moderate levels you’ll be just fine, but if you’re going for loud, you may be looking for trouble.

All the best.

JD