That would be a sympathetic resonance in the stand IMO. Sand would serve to remove that resonance somewhat, resulting in the altering of the sound, but IMO, the speaker should be decoupled completely from the stand because the stand should NOT be part of the sound. If it is, then it is adding something that should not be there. I use Auralex MoPads to decouple my speakers from the stands like what you see in the second pic with the table.
Stands do not remove cabinet resonance. If you don't want cabinet resonance, then you get a speaker that minimizes it, not a stand.