Threshold "T" vs. "S" - whats the differences

The S and SA series were designed by Nelson Pass when he was a co-owner of Threshold. The SA3.9e and SA-4e series in particular were great sounding class A amps.
The T-series were designed after Nelson left Threshold. They were also good sounding amps, but have the disadvantage to have been designed around the IGBT output semiconductors which are no longer availble if there are problems. I understand Threshold still exists as a service company and will completely rebuild the T series with new semiconductors.

Didn't the S stand for Statis? Also, the A series (their oldest) were pass designs. The 400A and 800A were great

Take a look at a following link:

http://www.rsvlonline.net/vintageamp/archiveamps.htm

Nice info about Threshold stuff from the first amp to the "T" series (with pics).