Going from the Adcom GFA 555 ii to the Bryston 4b SST or SST2?

I’m using the Proac Studio 100s! I’m looking to buy either a SST or SST2 used. 4b3  costs a lot more, even used. What kind of differences would I hear? 

I have all three amps. I believe you will be pleasantly pleased with the SST2, and much more so with the 4B3. Although I do not own a 4B3, I upgraded my pair of 7BSST2's to 7B3's, and was quite happy with that investment.

Happy Listening!

Thanks everyone for the great replies! Will look for an SST2

fyi, The Adcom GFA 555 II is very good high current amp from the 90’s.  I have a GFA 585 and a GFA 5503 for my home theater setup.  I am the original owner of both of them.  They have been in service since new.  The caps went bad on the GFA 585 and the GFA 5503 also had to be serviced about 10 years ago.  For me, the sound of your 555 II will directly correlate to the condition of the vintage caps inside.  If your GFA 555 II is running to spec, then, it will be hard to hear a difference with a similar rated amp at modest volume.   If it is not running to spec, it will be easy to beat with almost any decent running amp.  

I am a long time ProAc owner. I'd be concerned that the Bryston (my experience is with older Bryston amps) amps might be a little on the bright side for the ProAcs. I use a Cary CAD 200 Solid State. The amp is neutral toward warm sounding and matches my ProAcs well. I tried lots of amps that were neutral to bright (in the treble). None of them were a good match for my ProAcs.

Enjoy