Sorry for the late arrival...
Subwoofer 102 goes like this:
You need multiple subs (2 minimum, 4 is recommended) placed around the room to cancel out standing waves (below roughly 140 Hz) that otherwise are present in every room. Standing waves (depending on the room, where you listen, and frequency) will add or subtract 20 dB of output.
Check out Floyd E. Toole (worked at the Canadian National Research Council, involved in much of the research), Schroeder frequency (the 140 Hz mentioned above), and swarm (here at AC, a vendor term).