[MatrixDweller]

In principal if you bind a speaker to something super heavy, it should stop the enclosure from moving (vibrating) as much. This gives the woofer a solid platform to push in and out and not wobble back and forth minutely. Some speakers, like Von Schweikert or Wharfedale, say that filling the base with sand or lead shot will improve the speakers performance.

The added density of the base also helps disapate vibrations from the speaker, and from the floor coming back into the speaker. Spikes can also server that purpose and add to the stands rigidness to stick to the floor and not move.

Concrete is nice and dense, so is hardwood, stone, steel, or tubular steel filled with lead shot or sand. Concete blocks being the least expensive solution.

If you wanted something more appealing and like the concrete idea you buy a bag of premixed cement (with sand and gravel), mix it up and pour it into a mold. You could use a sonotube cut to the right size, a mold made out of wood, or a thick waxed cardboard produce box from a grocery store. After it has cured for a day or so you take the mold off and file down any imperfections. Other option are to get a slab of granite, limestone or marble cut to size, or take some tiles, make a box and fill it with sand or cement.