There are some articles on the interwebs that state that if you can't do "the crawl" as to where things should be placed for minimizing room modes (peaks and nulls).
I looked this up when I added my second sub.
The #1 suggestion was one sub dead center in front and one sub dead center of the back wall.
Obviously if you have a TV setup, that can be difficult. For me, I just didn't like the look of the sub on the back wall. It also was a bit odd for me because I have 2 rows of seats and my large 4cuft sub is downfiring, meaning that the woofer is "walled in" on 2 sides by, well, a wall and 12" of riser (plus the seats).
The 2nd best option was to put them both up front on the main wall, about 1/3 of the width of the wall in from the sides. This may force some to bump their front left and right speakers out to make room for the subs. That all assumes a reasonably sized room and not something with a 20+ foot wide front wall.
I went with option #2 because it also let me hide everything up front behind my false wall and screen.
I don't recall what a 3 sub configuration looked like. A 4 sub setup was 1 sub centered on each of the 4 walls. That is supposed to be "ideal" and no sub crawl required, but who has space for 4 subs. LOL (I joke because I know some people do.)
