I have the same stands you're talking about in the 24" height for my KEF LS50's. I filled mine with 'play sand' from Home Depot. This sand is clean, dry and very fine.
Regardless of what another poster in the thread suggested, you want to fill the stands completely to the top because you want them to be as acoustically dead and stable as possible. Even though these stands are bolted together, I've found that the base and top fit so well that none of the sand can escape through the joints.
Regardless of what another poster in the thread suggested, you want to fill the stands completely to the top because you want them to be as acoustically dead and stable as possible. Even though these stands are bolted together, I've found that the base and top fit so well that none of the sand can escape through the joints.