There seem to be some specialized sandbags available which might fit, or they might still be too big:
http://snakesandbag.com/My first thought was finding a bunch of the thin plastic bags you see in the fruit & vegetable area of supermarkets, then putting a small amount of sand in each one and dropping them down the tube.
"Fire in the hole !!"
You probably don't want to come up more than a foot or so (or maybe even less) since only weight down low is going to make the stands less tippy. Steel is a lot denser than sand, so if you are looking at it from a "tippiness" POV rather than "better bass" the Axiom weights suddenly make a *lot* of sense.
