Having a very similar set up to you in terms of room size, (i'm in a basement room that's 27 x 18 x 9) I would suggest going for the PSA v1811 and highly encourage you to get a second one down the road. The problem with that size of a room is pressurization, and material. I'm pretty sure you won't be able to pressurize that with one PSA v1811. You'll still get great bass, but definitely consider another one down the line.
If you're sticking with one sub, go with an 18'' over a 15''.
Subwoofers: 2x JTR Captivator 2400ULF Speakers: 4x PSA MT-110, 1x PSA MTM-210C, 4x Klipsch RP-500SA
Receiver: Denon x4400h Projector: Epson 5040UB