The Monolith 12"s are pretty large. I had one next to my Hsu VTF-15h mk2, and it was nearly as large as the Hsu. It's one of the largest ported 12"s out there right now.
The idea that you need to charge the entire space to get adequate SPL at the listening position is mistaken. We actually have an article in the pipeline that addresses this misguided idea. As long as the listener isn't too far from the subwoofers, they can get hit with decent SPL, no matter the room size.
As for the OP, it looks to me like the SB-3000s might be the best choice for him. Those are real humdingers for their size, and if the OP is looking at those, that means size is a pretty serious consideration. The SB-3000s are pretty killer above 40Hz. Not stupendous below 40Hz, but no sub in that size range could be.