My room is 20 x 12 x 6.5 so if you do the math the volume is only off by about 1%. Of course the resonant modes are different because of the dimensional differences but pressurizing the room should be similar.

I have on Ultra-X12 and it has no problem pressurizing that room size.

So I guess I'm agreeing with the idea of adding some bass traps. I considered bass traps myself but my basement, I mean home theater has too many water pipes, gas pipes and heating ducts make it worth while. So if you are in a situation where bass traps don't package (or you just don't like the look) then a second sub can help smooth out your sound.