Ok, did a little experimenting last night. It's definitely the receiver/amp that is causing the hum and not the sub or the sub amp. As I mentioned, my receiver/amp is a tube amp, and when I unplugged it from the power source, the quiet hum on the sub stopped completely. However, with the receiver/amp still plugged into the power source, even with it turned off, the sub still hums. In other words, the receiver/amp is sending a continuous signal to the sub, which appears to the be the reason the sub is not shutting off.
I'm no electronics expert, but I guess my receiver/amp either leaks juice when turned off or it is not grounded properly.
Kaz
