The poly fiber fill is to absorb the sound waves so that they don�t have a chance to bounce off the walls of the box and cause resonance.

I bet if you pack a ton of it into the box will could change your air space a lot but I don�t think that was the intended use.

98.5 Black SVT with 34k miles Build #5683 Born on 4\15\1998 WR Headers GMK CAI Blaupunkt system fiberglass tweeter pods L.E.D. tail lights in the works