http://billfitzmaurice.info/forum/viewtopic.php?f=10&t=398
Sub Placement:
While it’s customary with PA to have speakers to either side of the stage, that’s usually not the best way to place subs. Subs work best when they’re placed either close together for mutual coupling, or spread very wide to cover large areas. The basic rule is to have them either less than a quarter-wavelength apart or more than two wavelengths apart for their pass band, which for 40 to 100 Hz means less than 2.8 feet or more than 56 feet. To understand why click on this link, which demonstrates the destructive interference between subs that are separated:
http://www.falstad.com/wavebox/