I was told to put Kitty Litter in my new speaker stands. Was he joking?

I use it with no issues. Readily available and cleaner than sand Imo. I woulda tried steel shot but couldn’t allocate it locally.

Expensive, but Atacama atabites are what I use. They are tiny bits of metal uniformly round and flat in shape. They are said to be better than sand and lock together once settled, creating a very dense damping medium for speaker stands. A bit hard to come by in the US, but can be had from UK dealers who ship to the US. Can sometimes be found on Ebay.

You can buy lead shot at Cabelas or Bass Pro. I used a mask (happened to have a few) and gloves, while filling with a funnel. A 25lbs bag was plenty for both stands. Music Direct sells a product by KEF that costs about $25.00 a bag. I believe it takes 2 bags per speaker stand. 

Look up Poly-Pellets weighted stuffing beads. They're widely available in craft stores and in various sizes.
These are the cleanest item I found for filling speaker stands.
They are good for adding mass, don't attract pests, have no issues with humidity, and probably dampen and/or prevent floor vibrations from getting to your gear.

Glass beads also available as well, if you feel you've got enough plastic, lead, sand, or kitty litter in your life already....

Kitty litter here is being used for the original intent, but it does hint @ a 'dual purpose' sub enclosure....they don't seem to mind bass lines... ;)