Has this sub always acted this way, or just recently? Another test would be to just try manually run the receivers test tone/pink noise from speaker to speaker/sub. You should definitely hear a difference as you move the gain control up or down on the sub. Some people setup their systems manually still and don't use things like Audyssey, by using an SPL meter and matching everything to something like 75dB's.
If it doesn't change, something is wrong. I assume you are using the sub pre-out coax jack on the receiver to the line-in coax on the sub? If you are original owner I think there is 5 yr warranty, but that probably has passed if it is 2006 model.
