Looking down into my S80 and S90 side by side, they both have square chambers. The Selmer Larry Teal has the round chamber. The Soloist model has the arch, I believe.
The only visual difference on the outside is that the S90 has a longer table. If I were to speculate, I'd say that the S90 has a larger chamber than the S80. I get a much more full, deep sound especially in the low register when I play it. Listeners noticed a big difference between the two, even after a few notes. I highly recommend the S90, even after playing the S80 for the last five years.