You can use either poly-fil or fiberglass but I suggest Acousti-Stuf. Madisound and Parts Express both sell Acousti-Stuf. The way I install it is as follows:
1. Weigh out 1 oz of Acousti-Stuf per speaker using an ounce scale. You can find ounce scales at places like Walmart, Kmart, or Bed,Bath,and Beyond in the house goods section.
2. Tease out the Acousti-Stuf so that it is fluffy.
3. Evenly fill the back half of the enclosure ( center brace back) from top to bottom. If you use Poly-Fil use the same method. I don't recommend fiberglass.
You might want to experiment with both Acousti- Stuf and Poly-Fil to see which one you like best.