The Buescher lines get confusing around this time frame. I believe that the S20 was the Super 400, which was the next iteration of the top Buescher tenor. The S-25, while still a good horn, was a step down from the S-20.
But having said that, many members have reported that they are very happy with the 400 horns from this time frame, and even into the early part of the post Selmer buyout years.
If you really want to know, the only real way to know is to play a few and see what you think.
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