My neighbor was given a HT system in a box w/sub. He has a yamaha a/v receiver 6.1. I hooked it up for him as I've done many in the past including my own. This system however has no built-in amp for the subwoofer! The instructions show to run the front mains into the hi out of the sub and the hi in of the sub to the mains of the receiver. It works except there is not much sound coming from the sub. Is there another way to run this sub? Or should I keep trying to adjust the speaker levels? Please Help. Thank You.