So what speakers are you placing on them? I would not necessarily jump to conclusions about the stands because of glass and maybe the tubes can be sand loaded?
I've got some Athena AS-B1 speakers on them. And I agree, I shouldn't jump to conclusions. It just shocked me how much of that metallic ring could be excited by knocking on the side of the speaker. These aren't the most inert speakers to begin with, but now they sound quite metallic. Take them off the stands and that goes away.
I'm going to work with the stands and see what happens. Sand loading the tubes should be effective. They're not setup for it so I'll have to come up with a way to keep the sand from leaking out.
What I'd really like to do is successfully capture the effect as an audio measurement while playing test signals. If I am really hearing those tubes twang during music, it should show up on a mic measurement, and I should be able to show before and after differences from treating the tubes with sand.