Chazzbo,
I have had 4B/4B-SST and 7B-ST/7B-SSTs. I agree with you. The change from going from the B series to the ST series was significant; the ST to SST series results in a much, much smaller change. If you read the stereophile review (for what it is worth) on the 7B-SSTs, the reviewer prefers some aspects of the 7B-ST over the 7B-SST.
IMO, the ST/SSTs are greatly improved over the B series. The SSTs are slightly different then the ST series. I prefer the SSTs, but there are others who may prefer the ST. I think the SST and STs sound so much alike, that the price differential may not be worth it (to everyone).