Suggestions for 'filling the port' of bass reflex

Fabric and upholstery shops generally carry the stuff, as it's used in pillows, quilts, etc. Glad that it worked for you. Happy listening!

If you want to, "tone it down a little"; Go to a fabric/upholstery shop and get some (loose-not "felt")bonded dacron(http://cgi.ebay.com/Upholstery-Bonded-Dacron-Quilting-Batting-Padding_W0QQitemZ350220762258QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item518ac8bc92&_trksid=p3286.c0.m14&_trkparms=65%3A12%7C66%3A2%7C39%3A1%7C72%3A1205%7C293%3A1%7C294%3A50). You can experiment with different densities, by rolling it tighter/more loosely, and damp the port to your tastes. Most other suggestions will turn your cabinet into an acoustic suspension design, and completely ruin the system designer's alignment. Have you tried lifting them off the floor and/or moving them away from any corners?

The bonded dacron that I suggested will turn some of the acoustic energy in the port to thermal energy, and thereby damp it's resonance. Done correctly(not crushed or tightly packed), it won't impede the woofer's motion. It certainly will not damage the system in any way.