Btw, I tried packing material under the speakers stands (50lbs large monitors + stands?), and after a couple of days it completely flattened the packing material and I'm pretty sure any audible difference it could have made lol So that might explain why it's not commonly used or recommended! Foam would likely be the same, maybe you could get away with it with smaller and lighter speakers, but anything semi heavy I think will just get compressed, and I'm guessing would become less effective. So something like denser foam; yoga mat might work though.
FWIW, I think that on my desk for studio monitors foam did make an audible difference, but for the larger monitors on stands with packing material, doubt I could tell the difference... It's likely minimal if any, and if you've not got room treatment for instance, pretty darn sure you couldn't hear the minute difference it could make. Making it sort of a last % tweak, very minor difference, when everything else is great.