If it is a grounding issue I was going to put a proper ground inside the sub, I was just using the rca jack for testing purposes.

When I unplug the sub cable from the sub and its still humming, would that indicate the issue is the sub's ground?
I would expect that if it's humming, once I ground it the humming should stop. Is that a fair assumption?

I'm really trying to avoid disconnecting everything if I can