I have owned the 4Bs of different varieties. And I have owned the 7BST, SST, and now the SST2. For me the most dramatic improvement has been the jump to the 7BSST2. It has managed, to my ears, to take the 7B series to a new level.
I hate using the word "musical" as it doesn't seem to say much. But the comparison I would make is kind of like the lingo used with headphones. The Sennheiser HD800 is very accurate and analytical. It digs out a lot of information from a recording. Great for audio pros, but accused by some reviewers, and me, as being too cold, too analytical. Respected but not much fun to listen to. I've had fun listening to all my Bryston amps, but some reviewers I recall criticized them for lacking something. I didn't really understand that until I got the 7BSST2. It, for me, is a whole other kind of Bryston amp.
I'm sorry that this is not very clear, but if I had a choice between the 7BSST and the squared version, I'd jump at the squared.
Dave