If the stand is full of sand, it is decoupled. The term refers to bass to room coupling. The floor (normally). You have sand filled stands? I’d doubt that any bass harmonics are transferring down the stand. If you want to tune the bass (increase) you "beard" the front of the stands. By increasing baffle width and height it can be, a nice sonic nugget. Takes a bit of tinkering though...You’ll get it.. Even cardboard will let you work out the W x H. Convex, Concave, Flat, or split baffle...
Regards
Regards
Regards
Regards