As someone who has been using subwoofers in 2 channel aduio( actually I should probably call it 2.1 audio) for a decade, I have to agree with Kevin Haskins comments on Page 2.
The thread has 6 pages, I posted the link to the first page and once you get there you can easily navigate to the following pages-THEY are directional..Kevin Haskins had some thoughts that he posted on Page 2, but there are some interesting posts all the way to Page 6.