Because I believe that even very low frequencies provide directional information, and because most subs are front firing, I've never accepted the "one-subwoofer-is-all-you-need" theory. And down-firing subs, although theoretically omnidirectional, always seemed to lack good transient response (which is very important to me since my mains are electrostats.)
Recently, I installed a single new Martin Logan Depth, for reasons which I explain in the comments to my system specs (under my ID "nsgarch") here on AgoN.
Recently, I installed a single new Martin Logan Depth, for reasons which I explain in the comments to my system specs (under my ID "nsgarch") here on AgoN.