Rick, the Peavey article is substantially correct(there's no reversed polarity involving the other speaker, however)about the well-known cancellation effects about a quarter wavelength from a nearby surface or a half wavelength from another speaker playing the same sound. This has been discussed here on several occasions over the past few years, especially by John Bridgman and me.
The fact that the mains are set "large" and the sub is "on" doesn't necessarily mean that that they're both playing the low bass frequencies with resulting reinforcement/cancellation effects at various frequencies. The sub won't be also playing the low bass from the mains unless the setting variously termed "+", "double", etc., is applied.
