When I play CD's, my subwoofer is no longer receiving any input from my Receiver since I hooked up an external amp. The sub has always been hooked up to the sub pre out on the receiver and has never had problems, it's working fine for any 5.1 DVD material. The only thing I did was to hook up the rest of the pre outs to the new amp. It's a Yamaha RX-v795. I have tried sending the LF output to Both the mains and sub, and only the sub. There is no other control on the recceiver to do anything with the sub.. Any ideas??