I have verified that the odd grinding noise when turning the knob from 0-1 is not the sub itself. It also made the noise using another subwoofer.
This means that either the cable on the knob is messed up (going to replace it with regular telephone cord wire and see if that helps), the knob itself, or the amp.
I'm going to turn the amp gain to 0 in a few and see if I get the grinding noise again. If I do, something must be up with the amp. If not, something with the knob or cable.