Bryston is a progressive company and its products follow an evolutionary path towards the ultimate objective of perfection - the exact duplication of what was recorded on the source material - with all its detail, dynamics etc. Hence, each iteration of the 4B has been an improvement over the previous model - Bryston as verified by my experience, has never taken steps backwards with their amplifiers with any key aspect of sonic performance. So each successive 4B model brings you closer to the truth in the recording - with less distortion, increased linearity, dynamics etc.
Each progressive step is generally small - it's refinement on the previous - as has been said by James - it's evolution rather than revolutionary steps.
The ST was the largest positive step in the 4B line but the SST is clearly as step in the positive direction. With the SST/2, all the benefits of the SST series are there but now you have got liquid smoothness when it's called for and slam when the music calls for it. The ST will sound harder than the SST and definitely harder than the SST/2 but this is not getting you closer to what was recorded, rather the opposite.
The SST/2 series has really put Bryston in a very positive light - there are no real shortcomings in it - no real places to say it's lacking.
All that said, if you own a 4B-ST you're on the right path - enjoy it all but you won't go wrong moving to the new models...it only gets better. You can make the move when you're ready to and when you can afford it but you won't regret it.
I had a 3B-ST for a few years, it did me proud, but when compared to my 4B-SST/2, it was in a different league. The 3B brought what I would call tension to the table - which might be perceived as agressiveness or what you are referring to as dynamics but the SST/2 has all the dynamics but delivers it in a more lifelike manner - you forget that there is an amp there and just focus on the music. It also brings much more to the table in terms of finer details in the recordings...you'll hear what you did not hear before.
That's just my piece on it - only my experience thus far, which you are welcome to consider or ignore but ultimately, if you've got a Bryston amp - be happy and enjoy and upgrade when it makes sense and only then.
