Well, I will be able to answer you better in the next week or so as I will be receiving my 4B-SST2. I have my 4B-ST for sale right now on Canuck AudioMart but if it is not gone by then I will have something to directly compare it to.
For me it will be a win/win situation as I am driving big Martin Logan Quests that like lots of power and the SST series is more powerful and better able to handle the demand. I must say however, that if it does sound better (which I believe it definately will!) then the 4B-ST I have now, I will be thrilled to death as it already sound fantastic and I used to have YBA gear before.
If Bryston spends all that time and money upgrading an amp, you can count on it being worth the effort! I will give you the comparison tests results soon.