This is about managing the energy - the vibrations - going from the speaker into the surface they're sitting on. If there's enough energy, then the shelf becomes a sort of speaker as it vibrates along with the speaker that's energising it.
Various materials have different properties depending on frequency. Some weird things can happen. Some of the weirdest are non-Newtonian liquids. Depending on how they're handled they can turn from liquid to solid and back.
LINK. On the same vein, a material that feels slightly squashy pressed between the fingers can exhibit different properties - becoming almost solid - at much higher frequencies. The little silicon rubber feet might be a case in point.
Speaker isolation is a common topic for home- and studio- recording/mixing. Foam wedges can be used to both isolate and angle a speaker. These might not be exactly what you're looking for, but they're inexpensive and a start.
LINK. There are, of course, the Hi-Fi industry's solutions such as IsoAcoustics pucks. They're pricey, and the results can depend on a lot of other factors.
I would start with searching YouTube for videos of tests where someone has used a calibrated mic to plot the before and after results. Or at least find web sites that have done the same. What you're looking to see is a much cleaner frequency sweep and a waterfall plot that shows much quicker energy decay. If you can find 2 or 3 or 4 independent tests that all come to similar conclusions then that's likely to be a product worth investigating.