Hi,
As Dr.House has pointed out, you should increase the AV receiver sub level trim towards the -2 or 0-dB mark, so that the subwoofer gets enough voltage from the AV receiver to ensure that the auto-on signal-sensing function will kick in and activate the subwoofer, especially with low-level music CDs.
You certainly never want to run the sub's own volume control at maximum. Find a kind of happy medium between the sub's volume control and the AV receiver's sub level adjustment.
Regards,
Alan