How much lead/sand to fill stand rack?

Fill your stands enough to make them good and stable with your speakers on them. I promise you they'll sound great.

I dont know the stands, but if they are metal, you want to fill them to damp the ringing. I think they're probably heavy enough. My dealer recommended, and I use, Jonny Cat kitty litter, unscented. Cheap, fill them up all the way because it doesnt cost or weigh much, and it works great. If you really want them to be heavier than they are, mix shot or sand with the kitty litter and fill them up all the way.

That said, some people have reported that their speakers sound better with the stands only partially filled. I dont why that would be, but you could just experiment.

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.

Don’t forget that what ever you use to fill your speaker stand column with, it needs to be dry (not moist or wet). Damp fill can be dried out in an oven on a big cookie sheet with sides.