The problem with subs is it is impossible to get uniform, even and powerful deep bass with just one or even two subs. Also they cannot be located symmetrically the way the two main speakers are or you only make the inherent uneven response even worse. A minimum of three (and four is much better) located asymmetrically are required. The good news is using 4 allows them to be quite a bit smaller. The best solution seems to be here: http://www.audiokinesis.com/the-swarm-subwoofer-system-1.html
