I've built my own welded steel stands for a pair of monitors that I used to own. I became quite anal about what to fill them with, because I welded the columns closed after they were filled. I've also filled the cavities in the bottoms of my current speakers with sand as well.
Here's my accumulated wisdom/experience with mass loading and damping fillers:
Don't use lead. It's much more toxic than you think.
Steel shot works, but is expensive.
Kitty litter tends to absorb moisture from the air. In a humid climate, you can end up with rust inside the tubes.
Concrete mix. Even dry, it tends to be corrosive. Not really a good idea. Same sort of issues as kitty litter, too.
Playground sand works great. Make sure the bag is nice and dry and at room temperature. It's cheap, around $3-5 for 40 lbs. It is dusty though. Use it where you have good ventilation, prefferably outdoors. Remember that the dust is abrasive. carefully vacuum spilled and loose sand/dust up and wipe everything with a damp cloth after filling.
Quartz (white) sand, like is used to fill ashtrays in upscale hotels. Harder to find, but doesn't have the dust that ordinary sand has.
Whatever you use, tap the tubes as you fill them and let them sit for a day. Tap them every time you walk by to compact the filler a little more. Top them up and plug fill holes firmly. Taping the plugs in place is generally a good idea.
Hope this is of some sort of use.
Graham