What will F3 be on the 12 inch based speaker?
Could this be a crossover product with Hoyt-Bedford?
Will the width match the 3T and 3E?
Is your intention for it to be a "bass augmenter" (high F3), or subwoofer for music (F3 around 30 Hz), or subwoofer for music and HT (lower F3)? Years ago I met you at the Midwest Audiofest and your smaller offerings (at the time Fostex FE127 based I believe) struck me as ideal candidates for subwoofer integration. Had a Fried setup 30+ years ago made up of a LS3/5a clone with large passive mass loaded transmission line (MLTL) bass cabinets used as stands (114 dB at 17 Hz, too much for any residentially sized room but sounded magnificent in a chapel). Also enjoyed a similar but much more modest setup ("Gini" - LS3/5a clone with passive MLTL bass cabinet) a few years back and it loaded the room very nicely like almost no other regardless of cost at another audiofest.
Hoping you decide to go active (take load off small amps, allow more flexible locations using 3rd party wireless connections).
Have you looked into Floyd E. Toole's "Sound Reproduction" where he explains how bass waves propagate in rooms and the need for multiple (ideally 3 or 4) subs? His theories would support separate sub cutoff around 140 Hz for most rooms. Of course the THX standard is 80 Hz which is also the upper limit most recommend that the ear cannot localize. A couple of others here/around AC promote this "swarm" concept of multiple subs.