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

Personally, I would not do it.  My experience (repairing) with Bridging amplifiers is they become very unstable when powering low impedance speakers and driving them hard.  If a 16 y.o. tells you to "turn it down", I suspect you are driving them hard.  States nominal 4 ohms.  Some speakers dip below the 4 ohms.  Especially on the bottom end.  You will be lucky to just blow fuses.

But hey!  Sacrifices must be made in order to Rock and Roll.   It's only electronic gear.  Not like it is the last pair of amplifiers in existence. With trepidation I would say go for it.  Rock On!   LOL  I just remembered, the last bridge set I blew, I was playing Mahler.  I don't know why I assumed Rock.

No, there's nothing precious about the Adcoms.  I have them mostly as backups or to run stuff I'm testing.  

But yes, mostly rock.  I've been spending more time listening to my (new to me) KEF R11s.

I think I've decided not to bridge the Adcoms. 

I wonder why so many amps are bridgeable when most of the inefficient speakers are 4 ohms anyway. 

Run  some fans.

I had a Krell Ksa250  running B&w 801#3's

Got a Pair of Adcom 565 monos on loan from a steroshop.

Adcoms did very well, Sounding a Bit steely on highs but the lows were neck in neck.

The adcom will pry need to be checked for DC offset and or caps replaced.

Fine amps for the money

Which do y'all think will sound better with KEF R11s:  a single Classe CA-200 or 2 Adcom GFA-555's in vertical passive biamp?

In a somewhat analogous situation, I ran a pair of Hafler DH-220s bridged into a pair of Acoustat 1+1 and RHLabs sub. I thought  it sounded fine until someone smarter than me pointed out that the amps were likely quite unstable in that configuration. So as an experiment, I switched them out of bridged mode and used only one channel of each amp to drive the speakers.

I could not believe my ears at the improvement! Even with "less" power the sound was much better, more cohesive top to the now much better bottom. Others here have explained it better than I could but the bottom line is, I have practical experience to say that bridged is NOT the way to go in a low ohm situation.

Happy Listening!