If the stools have metal legs, try this: Turn them upside-down. Remove the end pieces (that close off the ends of the metal leg tubes). Fill legs with sand. vibrate/shake/tap to settle the sand, and top it up. Repeat a few times until the legs are well filled. Replace end pieces. Make sure the replaced ends are secure; add glue if necessary. Turn the whole thing back right-side up.
Now you have heavier, sand-damped speaker stands. Obviously, this only works for some kinds of stools; you can't fill the legs of wooden stools!
I've used this approach, as well as some nice Chinese hardwood stools as stands before. It can work very well, and if you get the right woodgrain, look very good, too, but it requires picking appropriate stools. Lightweight or overly resonant ones won't work quite as well as "real" stands.
Another thing that can improve a stool as a stand is to put a heavy slab of stone or a paving block on top of the stool, under the speaker, to help add mass and stabilize it. The speaker can be spiked to, or blu-tacked to, the stone block, which can be covered with decorative cloth for higher WAF.
