Chances are that it's reciever and not the speakers. While many say the Acoustimass speakers are terrible, they should still play in the rears. Alot of older recievers had VERY little power going to the back channels and you could hardly hear anything coming out of them.

Are you sure it's connected correctly? I wouldn't imagine replacing the rear channels if you just bought the AM-15s for that price. Is there a reason you can't take them back if you don't like them.