My theater room is 4,000cubic feet without a rear wall which really translates into a space of up to 8,200cubic feet since it opens to a larger area. I have 2-Vienna Mahlers, 1-Oratorio & 4-Waltz's. I planned on one REL Stadium III. Is one enough or is that the right sub? Please no DIY or internet-brand(s) advice. Would I be better off with a larger 18" like the Velo HGS18? or two? Thanks!