Your reciever is probably set for a pro logic or DTS mode which only sends signals to the sub that are encoded for movies, don't know because I don't know your reciever. It sounds like you are using the "subwoofer out" jack on the reciever. If your reciever has a "7 channel stereo" type mode it will probably solve the problem. Select the 7 channel stereo mode for the CD input. Most recievers will remember the mode that they are in depending on which input is selected (they remember the last mode selected for each input) if none of this make s sense or doesn't work, you can always send the sub a channel of high level or speaker line input providing it has that capability. This will ensure that low frequencie signals appear at the sub all the time