Interesting, I too am considering Tuba HT's for my theater, I purchased a pair of Eminence Lab 12's with the idea of building the Pro-sound LAB subwoofer unitl I came across the Tuba... Now I am thinking of 2 THT's 30" wide for each side of the screen.
The only other sub I am still considering is a Tapped horn using these same drivers, again one per box, they are not as efficient as the THT but go down to ~ 16 Hz...
From what I have found a pair of THT's in my room (21x16x8) is considered over kill, and that's what I want no feeling that I should have done more...