I just recently set up my system; Sony KDF-50WE655 LCD tv, Denon 2805 receiver, Denon 755 DVD and Mission FS2-AV 5.1 surround sound. There's sound through all speakers but sounds like a tin box as I am getting no response from the sub. I have all speakers wired to sub first, then connected to receiver via Missions umbilical cable, power is on, and sub in on umbilical cord (rca) is connected to the sub connection on the back of the receiver. Nothing. Any clues on how to proceed?