While I'm waiting on all my parts and stuff for a head gasket job I'm troubleshooting a few other small issues.

I have the HSE Harmon Kardon audio and the previous owner put in a nice Kenwood head unit with Bluetooth. All the door speakers/tweeters work and sound great but the rear stock subwoofers have only a slight amount of sound coming from them (they are in good shape as well).
The previous owner obviously had an aftermarket subwoofer in the rear as there are monster cables with power and output nicely wound up in the cargo area. I can see they tapped into the rear subwoofer wires in the little hatch area in the rear passenger side plastic bin area to the wires that go to the stock subwoofer in the rear door (the wire colors match). They just tapped into the wires and didn't cut off the wires.

I know I should have more sound coming from the rear subwoofers, I've tried all the settings on the aftermarket head unit without any change. Is there anything else I should look for? Is it standard practice to do something to the stock subwoofers when an aftermarket head unit and amp are installed?

Thanks all.