I personally preferred the SST series over the ST. Like others here I prefer the added smoothness and refinement and found them more neutral and less bright. But there are people who preferred the "hotter" balance of the ST series and found them to be more dynamic and involving.
I think the best thing to do is to compare an ST amp with their subsequent SST replacement. But of the amps you mentioned, there's no question to me that the 7BST will be the best amp. First of all, you won't find too many people who actually think the SST series to be a quantum leap in advancement. And second of all, the 7B will be much more dynamic and throw a much deeper and wider soundstage than both the 4B and the 6B regardless of whether it's ST or SST. The 7B is simply a better amp.
I think the best thing to do is to compare an ST amp with their subsequent SST replacement. But of the amps you mentioned, there's no question to me that the 7BST will be the best amp. First of all, you won't find too many people who actually think the SST series to be a quantum leap in advancement. And second of all, the 7B will be much more dynamic and throw a much deeper and wider soundstage than both the 4B and the 6B regardless of whether it's ST or SST. The 7B is simply a better amp.