Two subs allow you to cancel out at least one room mode. Much easier from a placement standpoint. Harmon International has a very good paper on this by Floyd Toole and Todd Welti.
One or two JL subs?
I am looking at upgrading my HT system by adding new sub(s). I've decided to go with JL subs but am trying to decide which route to go. Two 112's vs a single 113. The room is 12.5 x 26 x 7.5h. I sit 12 feet from the subs. I know that JL prefers a two sub setup. But who wouldn't if you can sell two subs vs one? BTW i am not into extreme SPL's i listen to movies and music at sane levels. I would be most interested in what JL owners have to say. (Respectfully, i am not interested in in opinions about other brands of subs, i am pretty set on the JL's)