Replacing the OEM HK speakers

My wife and I purchased an ‘09 Subaru Outback with an OEM 440-watt Harman Kardon audio system. My parents only listened to talk radio in the car and never at loud volume levels but, somehow, the OEM 8”sub woofer is blown. So, I purchased a Pioneer TS-SW2002D2 8-inch Shallow-Mount Subwoofer which is 2 ohm DVC Pioneer sub that can be used in a “Free Air / Infinite Baffle” design. The OEM amp is a complex little device. In the Legacy sedan, the OEM sub is a SVC. In the Outback wagon, the sub is a DVC. So, in the sedan, only one output on the OEM amp is used for the sub but the wagon uses both of the outputs for the sub. The OEM amp also send signals to the separate tweeters in the doors and the separate woofers in the doors in the front while the back door speakers are full range with no separate components.

I got some generic dynamat to cover the door panels and I got some Polk Audio DB6502 6 1/2 separates for the front doors and some Polk Audio DB652 6 1/2 for the back doors. I’m not going to replace the OEM head unit yet or the OEM amp as it would be a PITA to rewire all of the speakers from an aftermarket head unit to an adtermarket amp. Also, I’ve hear that the OEM amp runs some other non-audio related functions for the vehicle and I don’t want to mess with that.

The OEM amp is under the passenger seat. The area under the driver seat is open if I want to add an aftermarket amp later. Adding an aftermarket head unit wouldn’t be super complex but I would have to rewire the head unit and speakers so the signals don’t go to the OEM amp.

Any thoughts or advice?