I would not put the subs on the same circuit as the AV equipment. They can be on their own circuit, 15 amp is probably fine but overkill is never bad if you want to go 20. Same with the AV equipment. I run my projector dedicated on a 15, the equipment on another 15 (5 devices) and the sub (and later on a second) on another 15... No problems at all.
You won't really know where to put the subs until you can get in there and do a crawl. Chances are good though you'll want one up front, and one in back. Two in back may work if you play with the phasing a bit.
