Bryston 7Bst vs 7B3

I am running a 4bsst2 as a centre after a previous 7bsst. The sound is better - slightly cleaner and tighter. The same difference when I moved from 14Bsst2 to 28bsst2 monoblocks (also better at lower volumes). I missed a trick here as I bought the 28bsst2s believing them to be the latest bees knees models, only finding out  6 months later of the sst3s! 
The comments I have seen say that there is a sonic improvement moving up from say 28bsst2s to 28bsst3s - but nobody has defined exactly what that is for us listeners. Easy to say, but not easy to define. I presume more of the same type of improvements moving from s to 2s
And is a 4b3 bridged better (and more flexible)  than a 7b3? Too many questions.

I have just put in a 4b3 instead of a 4bsst2
It is almost like a different amp - yes I know it is a different amp but you know what I mean!
Very dry sound (says me talking bollocks) and maybe losing a bit of airyiness or something - but so precise
I have about 50 hours on them so waiting for a (150+??)  burn in to see if they warm up a bit 
And yes the 4bsst2 is a worthwhile upgrade to the 4bsst