Which Damping Material For In-Wall Speakers?

I am about to install 18 11" round Polk LCi80 InWall speakers, and would appreciate any recommendations as to which damping material to use. I saw a product called AcoustaStuf in Parts Express, but the cost is very high...($35 for 5 pounds, which they note would cover a total of ten cubic feet).


Would polyfill or fiberglass work as well, or is there some other substance which would do the job. The speakers are high end, and the wall construction is typically modern sheetrock/studs.


Also, would placing a sheet of foam right behind each in wall, be recommended (to prevent any fill from getting into the speakers)?


Thanks in advance for any suggestions..