That's almost the dimensions of my closed HT (19x21x8). One 400 won't be enough and Ian suggests 4 of them to flatten the nodes and foregoing any equalization. Also sudden loud LFE can go well over 75db. Even though a single 400 is rated max SPL in room of 116db, and is quite powerful for its size, I would say you need more headroom and more subs.

John