Some of the latest thinking is to isolate elements of the system from vibration and microphonics and there seems to be strong evidence for this. Consider looking into feet that isolate the subs from the floor rather than using feet that couple to the floor. This may accomplish what you are looking for in a different way.
Mass-loading subwoofer
I have a Rel G1 MK2 subwoofer (108#) that sits on an Auralex Great Gramma V2 platform (300# limit) and I think that I want to mass-load it.
The question is mass-load with what and, how much weight?
I’ve been looking at (for instance) a box of granite/marble slabs etc. from Lowe’s that are still in the box and strapped, or bags of steel shot from sandbags.com etc. on Diversitech pads.
Would those options “rattle” etc?
I know that the obvious answer is to experiment but I really would like suggestions first and there seems to be surprisingly little information on the subject anyplace.
Thanks!