I owned a GFA 555 mk2 many years ago. I didn’t like it at all. The speakers at that time were JBL L112’s and Green Mountain Diamanté’s. I purchased the Adcom primarily to power some subs I had. Of course I had to hear it on the main speakers. The competition was a Classe DR8 which was admittedly a much more expensive amp. The Adcom lost. Dry and sterile was what I recollect.
I currently own an ST500. Wonderful amp for B&W speakers. I’ve always paired it with a tube preamp which is a wonderful combination. I prefer the ST500 to any of the class A/B amps I tried on the B&W’s. Previous Classe amps were a pair of DR8’s and a CA300.
It’s your system and your ears so let’s find which you prefer.