I own a 4B-ST and 4B-SST2, and both sound quite different. Sort of a good thing though, as they match better or worse with different speakers. The ST sounds more exciting, and maybe less refined. It's lived most its life powering my B&W 801s2's, and is a great match for them. The SST2 is too refined, and almost sounds compressed powering the 801's. I've just had the 20yr old 4BST repaired & recapped by Bryston, and it was really worth it! (~$700)
The SST2 drives my pair of Tannoy 15's, and the low noise pays off there. The ST is too harsh on the Tannoys. It's all about system matching! I borrowed a Parasound A23, and found it too bright. I've not heard the A21 in my home. I am SST3 curious, but not in the amp market now that my ST is restored!