Jon,
The Sunn speakers that were in my 1967 Solarus were 12" with huge ceramic magnets - even bigger magnets than those on Twin Reverbs.
Ya got two or three choices:
-- JBL 120's - loud and clean, plus the really cool aluminium dust covers.
-- Fender replacements, whatever they are using for "The Twin" or the '65 reissue. This might come very close to the original sound.
-- New, specialty speaker. (Weber, Reverend, Celestion) The advantage being earlier breakup, etc.
All this for guitar. I suspect that an open backed cab would not last long with a bass.