There are products in the car stereo realm for sound deadening like Dyna-mat and Hush-mat. Call me crazy, but I'll take a $32,000 or $42,000 car home the first day and Dyna-mat everything- headliner, floor, doors, trunk, and even the license plate-
I work for an air filter company. Polyester filters are the same material as poly-fil in comforters, jackets, and furniture padding. It makes a good fill for speaker boxes.
You ever hear of Alma Gates? Car stereo champion for years in her 60's and 70's-
2013 Classic 30 Limited
2007 Silver Toyota Tundra Crew Max Limited 5.7 iForce
2006 Vivid Black Harley-Davidson Road King Classic
1999 Black Nissan Pathfinder LE
WBCCI #1811, Region 6, Unit 56