Here is my setup for my home theater subwoofers. I have a old Harmon/Kardon AVR-25 receiver as my sub's amp. Connected to it are two Cerwin-Vega Re30 with the tweeters and midranges disconnected. So just the 12" woofers are connected. One "sub" is connect to the left channel output and the other to the right. The "subs" are up against a corner of my room facing out. One sub is directly on top of the other, making a "tower of power" as I call it.
My question is why do I get better, more powerful bass with them 180 degrees out of phase? I always thought that if speakers are in phase there output combines but if there 180 out of phase they try to cancel each other's output out.