Which subs are best for home theater?


I now have a double 12" self-powered sub that I'm using in the front channel for home theater. I am thinking of adding a rear-channel subwoofer. My dealer has suggested using a single 15" down firing sub in both front and rear channels. Would a 15" in the rear be overkill? Should I use a 15" for the front? And maybe use the double 12" sub for the rear channels?

Another question would be is a double 12" sub better on audio than the single 15" sub?

Thank you in advance for your suggestions.

MR.upgrade
mrupgrade
Marakanetz, does that accomplish the job of getting the air moving?

Do you find it difficult to integrate with the main course? Which direction does the main pressure come from, front or downward?

Hope these questions are not too technical.
albert: good questions. and i will add my own: are you, marakanetz, as i assume, also retromingent? -cfb
I would have to throw my vote for an Earthquake Supernova MKIV-15. It uses a 15" woofer and a 15" passive radiator, and can pump out extremely high levels of sound well below 20 Hz. I can sell these subwoofers for less than anyone on the net, so if you are interested, please let me know.