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.
Thanks for the reply, Alan. Your suggestion makes a lot of sense, however there is one problem.....it's used for stereo only, so no TV coax cable is hooked into the amp.
The only thing I can think of is that the amp is a tube amp which I'm wondering might have something to do with it. However, I would guess that whether it's a tube amp or solid state/transistor amp, when they're off, they're off, and there would be no signal either way. Might it be possible the tube amp, even when turned off, is somehow still leaking a tiny bit of juice through the speaker outputs?
I'm at work now, but when I get home tonight I will try experiment a little by unplugging the amp completely. I'll also go check out the troubleshooting article you recommended as well.
ps. thanks to all the others for their responses also.