If you have the ability, the Infinite Baffle sub systems are the way to go. The is a forum called "Infinitely Baffled" that has tons of info, and I did it in my room, which is a couple feet narrower than yours. I used (4) 15" drivers in 2 manifolds in the floor, the bass is better that any sub I ever owned (Velo, Def Tech, Earthquake, etc.) for movies, and music. I use them all the time. The effect of the floor shaking is the best for movies. Anything else for a dedicated theater is not going to be as good, IMHO.
You will not believe it, and the total system, which included the 4 drivers, 2 pro amps, 1 xover, and the wood for the manifolds was around $1000.
Good Luck !!