Filling audio rack tubes with sand, metal, grape nutz???

Whatever you use, make sure you are putting it into a sealed cavity that will not spill out if tipped over and will not allow the material to escape into your home environment.  Do not ingest or inhale the material while working with it and use appropriate respiratory protection.  Both lead and silica dust are known to pose health problems if ingested or inhaled so, whatever you use, study the MSDS sheet on the material before handling.
A few thoughts come to mind;
1. Sound Anchor uses sand in their stands
2. Some people mix lead shot with the sand (like shotgun reload material)
3. Starsound Technoligies swears by their steel microbearing stuff that they used to use to damp cables
http://starsoundtechnologies.com/microBearingFill.php
4. The micro bearing stuff in #3 may be nothing more than steel blasting shot in a small fancy container - this is about a third of the cost;
https://www.google.com/shopping/product/15404022574801836148?q=small+size+Steel+shot&client=safari&sa=X&hl=en-us&biw=1024&bih=666&output=search&prmd=sinv&prds=ds:1,num:4,cs:1&tbs=vw:l,ss:44&ved=0ahUKEwjunYChyefdAhXD44MKHVqVAqYQu4UCCA8

Excellent@mitch2 just ordered a bucket of metal!

I had a friend that poured a liquid gel type compound into the legs I think he got it from one of the hobby stores. they were dead to any taping and added a surprising amount of mass. of course there is no removing at that point your committed.  

There is a product that is used to solidify latex paint for disposal. You might be able to use that with a couple quarts of inexpensive or surplus latex paint. 

If you ordered the steel shot you will not have the same level of concern as with lead.  Of course, you can look to see if there is an MSDS for how to handle the material and if you want to be absolutely safe you can always fill and cap the stands outdoors so there is no residual dust in the house 😷 and then carry (or roll with a dolly) the stands into the house. Your biggest risk is to not hurt your back moving them around 🤕