I used a similar placement as deep 333's diagram, but I only have 2 subs (though each sub has 2-12" drivers. Rythmik G22's) so I placed them at the EXACT midpoint on each of the 2 long walls and this works extremely well. It would probably work even a little better if I could do the midpoint of each short wall but that's just not possible for me. My goal was to simply complement my main speakers (Focal Sopra 1) so that it sounded like all bass was coming from them and I'd say mission accomplished. I have the gain setting at about 25% on both so the subs are just loafing and the abundance of smooth bass is very satisfying. I'm actually planning to replace my Sopras soon, but I'll still use this sub set up with whatever speaker I end up with.
Of course, I realize that this arrangement would not work as well for those that don't have a dedicated room, and further, my room is shaped almost exactly like the diagram above, making it perfect for this kind of placement. But if it'll work in your particular room I'd highly recommend it.