Auralex SubDude

I used a similar Auralex Gramma under a Vel DD-15 for a time. It is effective in isolating the room from the sub, but not much good in communicating LF with precision. A Star Sound SP-1 with Audiopoints brass cups gives more delineation in the sub bass region and reduces muddiness that cabinet resonances will otherwise introduce into the upper bass and midrange.

Mateored, rather than lead shot, you might try a bag of microbearing steel from eBay or a foundry supply company. I use this inside the enclosure and in an ultrasuede bag placed on top of my modified Merlin VSMs. The Merlin is particularly active with vibration across the top of its cabinet. Microbearing steel is more reactive than lead shot in absorbing vibration and is of course non-toxic.