m-db and willemj, a sub and the room it is in are a system, they can not be separated. The listening room itself is a speaker enclosure, in a way, and the subwoofer "swarm" is a way to deal with room modes acoustically.
Low frequencies being omni-directional is often cited as a reason why stereo subs are not necessary. One thing that makes an OB/Dipole sub different from a monopole sub is that it "loads" the room differently. At and below a certain frequency (depending on the distance between the front and rear of the sub’s woofers), the wave from the front of the OB/Dipole and the one from the rear, both of which are omni-directional, meet at the side of the OB/Dipole H-frame (or W-frame) and, being of opposite polarity, cancel out each other. The same thing happens with planar loudspeakers, but at a higher frequency. This acoustical phenomenon creates a null to either side of the sub’s frame---there is no sound propagated by the sub in the sidewall-to-sidewall plane, and the frequencies related to that room dimension (it’s width) are not "excited" by the sub. That is one reason why the OB/Dipole sub is perceived as being "faster" than the monopole sub---with the OB/Dipole Sub exciting fewer modes, the room exhibits less resonance, overhang, and boom.
It is actually the room that is "faster" with an OB/Dipole sub, not the sub itself. But just as with "normal" subs, other factors affect the quality of sound. The GR Research/Rythmik Sub combines the attributes of OB/Dipole room loading with Servo-Feedback amp/woofers (a joint effort by GR Research’s Dannie Richie---well known for his OB speaker designs, and Rythmik’s Brian Ding, known for his subs, of course), and is the only sub of it’s kind in the world. It is available in the original dual-12" woofer version, and now with 8" woofers instead, for smaller rooms/lower SPL.