Hi There,
Heres the article
http://www.onhometheater.com/features/20040801.htm
And the White paper it links to...
Basicaly it says that with one sub you can get ideal listening for a single position, but for more than just a single sweet spot you need more subs, upto a max of four. Idealy placed in the corners, but two in the mid position (back and front) is almost as good, for a 'normal' room.
cya,
Lee