Regarding stereo subwoofers, Dr. Griesinger of Lexicon observed the psychoacoustic phenomenon where bass coming from directly in front of you is perceived as being inside your head instead of in front of it. To this end, he came up with something for the Lexicon processors called Bass Enhance, which takes the bass from the center channel and redirects it to side subs or full-range mains, plays a few games with the phasing, and in principle offers a significant advantage over bass without this technique.

In practice, this technique has been shown to be highly room- and position-dependent. It's an option on the processors, so some people (like myself) use it, and others do not.

Jeff