One of the things that I don't understand is that a lot are using a "pair" of subs. I was taught that bass is non directional. I am also a firm believer in crossing over between 80-90Hz. I find that going up over that allows voice into the sub, and that seems to be where problems begin. I've always used a single sub. Not saying it is right or wrong, just find my system to sound great and the sub blends in well.
In regards to putting multiple subs around the room (rather than stacking them), I believe if you could do a 3d graph of the listing area, it would be like a mountain range with peaks and valleys all over the pace.