Halcro, Todd Welti et al published a couple of thoroughly scientific (nothing "quasi" about them) papers on the same subject in the Journal of the Audio Engineering Society a few years ago. What you see at the link above is the version he wrote for non-engineers.
Another very scientific proponent of multiple subwoofers is Earl Geddes, though he hasn't published any peer-reviewed papers on the subject. His approach differs from that described in the Welti papers somewhat; Earl advocates asymmetrical placement. Here is a brief study he did comparing four-corners placement with asymmetrical placement of four subs:
http://www.gedlee.com/downloads/sub%20study%20.pdf
I use a simplified quasi-Geddes placement strategy; I do not elevate one of the subs as he recommends.
Duke
Another very scientific proponent of multiple subwoofers is Earl Geddes, though he hasn't published any peer-reviewed papers on the subject. His approach differs from that described in the Welti papers somewhat; Earl advocates asymmetrical placement. Here is a brief study he did comparing four-corners placement with asymmetrical placement of four subs:
http://www.gedlee.com/downloads/sub%20study%20.pdf
I use a simplified quasi-Geddes placement strategy; I do not elevate one of the subs as he recommends.
Duke