From what I can tell, this is a full range speaker with a built-in powered “subs?” I’m not sure I’d to go with a generic off-the-shelf amplifier. It’s possible they have incorporated some equalization and limiting specific to the design. I’d check with Polk and see if they have replacements. If not, you’re out of luck and will have to find one used. Try ebay.
Regards,
Wayne A. Pflughaupt