Re: Adding Polyfill to the stock Subwoofer enclosure to make it sound better.
If I were to tear it all apart, I'd do a whole lot more than that. I'd add more mass to the enclosure (bedliner, sound deadening, fiberflass, etc.), seal it, add polyfill, replace the driver with a real subwoofer, add an inexpensive mono amp, and button it back up. You have to nearly rip the whole rear interior apart to get at it. Adding polyfill is not going to make a difference worth that task.
I haven't heard the Ford Sony systems, but this worhtless OE Alpine sub isn't worth messing with. Most leave it, or replace it.
2014 Durango Limited AWD in Granite: Premium Package, 2nd Row Captain Chairs, 2nd row Console.