Hello Kaz,
Your remark that there is a "quiet hum" present when the EP175 has not shut off suggests to me that you have a ground loop hum. The EP175 would perceive this as a signal input even if your main AV receiver or amplifier is shut off. Ground loop hum can exist even if all the other components in your system are turned off. Check the cable-TV input. It's usually the source. Disconnect it where it enters your cable/converter box. If the quiet hum disappears, then you'll need an isolating transformer available from Axiom or (rarely) from your TV cable provider.
Try our trouble-shooting steps to getting rid of the ground loop hum (in the Articles Archive) and it should solve the problem of the sub not turning off.
As JohnK pointed out, the auto circuit operation is not dependent on using the low-level or high-level inputs.
Regards,
Alan
