Don't get a center channel that's weak compared to you L/R's. Maybe keep the RTi8's and upgrade the center to the CSi5?
At a similar price point to the Polk sub, an Hsu STF-1 or STF-2 would probably provide more satisfaction, but moving up to the HSU VTF-2 MK3 or SVS PB12-NSD for the marginal extra cost would offer huge benefits, even in a small room. Heck, I've got the PB12-Plus in an 11 x 14 room and love it.