the reason that I suggested the ported with 17hz tuning is that gives more than double the output of sealed subs in the very low notes (16-20hz), while also keeping maximum blasting authority up in the 30hz region which is the wheelhouse for movie bass.
the lower the tuning frequency for a ported sub, the less output higher up in terms of max spl, so it is a tradeoff.
the si 18", ~12 cubic feet, tuned to ~17hz, in a large room is a REALLY good balance between max output, max extension, and system value.
there is also the possibility of an infinite baffle subwoofer system, but that would be more cost and size.
sounds like a fun project!
as for the tht and f20, the f20 has a lower tuning, so you get more extension. i'm not exactly sure what the tradeoff is in terms of spl higher up and i'm too lazy to pop hornresp open at the moment. :-)