Eminence Engineer's rule of thumb for Polyfill pillow stuffing for speaker boxes is:
Minimal Fill is 1/4 lb per cubic ft of box volume.
Normal Fill is 1/2 lb per cubic ft of box volume.
Heavy Fill is 3/4 lb per cubic ft of box volume.
Our engineers never go much more than Heavy or it just gets too dead sounding and it just does not sound good.