I manage a drapery and upholstery shop. Don't waste your money on "sub box polyfill." Follow crutchfields recommendations, but until they stop shafting us buy it from fabric stores, craft stores are ALWAYS more expensive, there is a difference. You can use "polyfill" if you want to stuff your box with fluff or "batting" comes in sheets of different weights (thicknesses and densities) to line your box in. Just march in to that flowered, potpourri, no mans land and ask for it.
Ok what polyfill does is that it slows down the sound waves in the box basically. It doesn't absorb the sound since its so light, and that makes your bass sound deeper, it doesn't make it sound louder per se, but you can get a few Hz lower out of your box. It does work, but only on sealed boxes, if you have ported or bandpass boxes don't even waste your time with it.