+1 on two subs over one. Aside from smoothing in-room frequency response, it removes localization, if you place them next to each of your front mains. You can also raise the crossover without localization issues as well. Something else is I have to wonder how well the SB13 Ultra would fair against the F113. You can get two of those for the price of one F113, I can not see how that wouldn't be a more preferable system. Also multiple Hsu ULS-15 and Rythmik F15HP. Hell for $2.7k you could get a Seaton Submersive, and for just a bit more you can get a Funk Audio 18.0.