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I imagine blue tac would actually be quite good, it has a certain viscosity so would be a really good absorber.. sand is better than concrete both in isolating the sound source ( there isnt any thing 100 solid for sound waves to pass through) and the small particles deflect the waves as they move through the sand.
Moserism-
Concrete has the advantage of high inertia and rigidity - surely a wooden box containing the sand would reverberate, necessitating an acoustically 'dead' filler to give it mass and an element of panel damping?
If the blu tac (or equivalent viscoelastic body) is effective as a vibration isolator, then the acoustic qualities of the stand will probably be less important than it's structural stiffness
Having said that, it would be difficult to make concrete look attractive...
MrE
20kg concrete blocks with blu tac to hold the speakers in place works well ?