what else could be fill inside boxes?

Back in my "ghetto days", I would actually stuff a full-sized bed pillow into each side of my sub box.  I figured nobody would see it.  But then I quit smoking herbs and my brain cells started to come back. 

Last time I bought a pair of subs from Crutchfield (you know, back when nobody could beat their prices, unlike today when everyone can beat their prices!) they always said to use "Quilting Dacron" which they said was available at fabric stores and craft stores.  It was the stuff made to line quilts when they got sewn together.  Sure enough, I found it at Hancook and Joann Fabric stores.

Now days, if you go into a fabric store and ask for Quilting Dacron, they point you to the Polyester (aka "polyfill").  It's pretty cheap too.  But you have to cut it in half.  One big bag would work for 2 sides of a sub box.  It's what I use in my present "generic box" that I bought from Car Toys (a popular electronics retailer over here).

As for the "reason" to use polyester, I can't explain the technical stuff.  But I can say it stops the hollow, echo affect.  And you WILL notice a difference!  Just to hear the difference, buy the poly fill first, then mount your subs without it.  Turn the system on and listen.  Then shut er down and stuff the polyester into the sub box, and  try it again.  Hear the difference?  Feel the difference?  There is a difference.