Filling hollow tubes of my component racks

YES. Well , maybe. I am a dealer for Star Sound's Micro Bearings and this is a danger with them, they are exceedingly dense, much more than Atabits which I also handle. After I think about it you will probably not be able to get enough in to over damp it. I doubt if you will get enough gravel to do any good, I used cat litter for a while but abandoned it. I put 2 quarts of Micro Bearings in the target stands I use for my speakers [ one in each] and it works quite well, also have it is my Star Sound rack , I think it would work well in yours as it is very dense and heavy, think steel sand. But just start with a quart in each, they use to fill speaker stands full with it and I listened to one of them. it did "kill" the sound. Someone advised using steel blasting shot, I have never done this but it seems reasonable as the Atabits appear to be small steel shot and they work, but not as good as the Micro Bearings. I would give the gravel a pass.

I used cat litter in my Skylan stands but didn't really seem to do much so I took it out. I couldn't get the Star Sound for a long time because their supplier went out of business suddenly and they had to find another. I have not done a scientific test because the Micro Bearings are so small I only want to put them in welded stands. Right now I have them in 15" Target stands I use with Spendor SP1/2s. I have the Atabits in a pair of 20" Atacama stands I use with Mini Utopia Focal speakers. I like the Micro Bearings because they seem to be the most effective is damping the resonance of the steel stands, VERY heavy for their volume. I was using lead shot but I thought it caused a coloration in the sound I don't hear with the MBs. Just my impressions. The MB seem even heavier than the same volume of lead shot, I haven't measured this and both are heavy. Lead is, of course, considerably heavier than steel but the MBs are much smaller than the lead shot so you can gat many more into the same volume.

The basic question in my mind is how they act to damp vibrations. With very dense material of the smallest possible diameter the waves will be propagated to the smallest distance before being dissipated. I think that is how the "sound" of the material comes about. I did not think that that cat litter I was using was especially effective at absorbing vibrations so I quit using it. I would question whether the stone would be very effective either but the expense should not be too great if you want to try it. I have no problem using lead, did so for years and still have some but like the others better. I would check out the steel blasting shot if you are in an area where it would be readily available. Get the smallest diameter if you get some.