The first Gen Model T's (1969 - 1973?) used a bassman/plexi style tone stack.
I'm not nearly as familiar with the Hartzell era as I am with the Sundholm era, but I'm pretty sure that there
wasn't a Model T until 1973 at the very earliest. I'm not sure that this "first Gen" of which you speak actually exists; at least I can't find any
documentation that it existed. Of course, there were probably a couple of prototypes of the reworked 4 X 6550 chassis that
became the Model T around before 1973, but I don't think you could
buy one until 1973.
AFAIK the Sunn tube model lineup was pretty much three basic circuits until 1972, the 2 X EL34, 2 X KT88/6550, and 4 X KT88/6550. The EL34 amps went away in 1970, and the remaining two circuits were SS rectified and output rating was increased. By 1972, the entire tube amp spectrum was reduced to a bass and guitar version of the 2 X 6550 (190B, 190L), and a bass and guitar version of the 4 X 6550 amp (350B, 350L).
If you think about it, it wouldn't made sense to have a Model T in 1969-1972; it would have been counterproductive for them to market a
different 4 X 6550 that would compete with their existing 1000/2000 series amps,
particularly when the trend for
all amplifiers was heading towards "all transistor all the time" like it was in the early 70's.
I think the Model T was a way to cut down the remaining four tube amps (190L,190B,350L,350B) to just
one multipurpose amp for guitar and bass for 1973.