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

Currently I use macbook -> apollo twin x -> adcom gfa 555 ii -> proacs. 

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? 

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

Thanks for the advice everyone! Will explore other brands as well. Looking forward to the research

How would the 3b sst2 be? I do nearfield listening anywhere from 50-86 spl/db.

People online say that the Brystons are a great match for the ProAcs. The person I bought the Adcom from said it was in great condition.