How much lead/sand to fill stand rack?

This may not be worth the trouble (it may not be easily done with larger stands like yours), but I have filled several smaller diameter metal speaker stands with sand/lead in this way. If you strike a medium sized tuning fork and place it in contact with a short length of metal tube (1"to 2" diameter) containing 2 to 3 inches of sand, you should notice the sand furioulsy vibrating/moving in reaction to the tuning fork. This shallow depth of sand allows for this movement, which transforms this energy into heat. If the sand depth is too deep, I have observed less movement in the sand, likely due to the larger mass and settling of the sand. To maintain this type of energy transfer, I have layered sand, with small "spacers" made of high-density foam-rubber, so that each layer of sand has a small amount of space between layers. This allows each pocket of sand to vibrate more freely. Also, I place an amount of lead shot in the bottom of the stands for stability. I also use this construction method, i.e., partially filled layers of honeycombed "pockets of sand" in my home-made amp and sub-woofer stands, which works better than mass-loading alone in my trials. Good Luck.