About Bryston - their trade-in program only applies to Bryston gear. So if you're upgrading from a 4B to another Bryston piece they will take your 4B in trade.
Different generations of Bryston sound rather different. The SST series is a bit mellow and very pleasing, the SST squared is little bright (so I'm told), and the SST cubed is much more precise and a little mellow (so I'm told).
Anyway, I had a 2B which was used for a sub - and it was good. I also had a 3B SST and a 4B SST for my Magnepan 2-channel system. Then I went ESL with DIY electronics for 2-channel, and sold the Brystons.
But now I have a separate HT which requires good clean subs, and that means a big, stable, bullet-proof, forever SS amp: that is, a Bryston 4B SST, bought factory reconditioned and certified from Bryston.