The stillpoints way of looking at things is reverse to this and in my experience is the best.
You dont stop vibration getting in as much as you drain it away - dont lock it in using the wrong thing.
Anything rubber orientated tennis ball for example is rubber its locking any internal vibration. In contrast other companies use wood, aluminium and other materials.
In my testing I tried different materials - plastic, aluminium and stainless steel - and stainless steel is by far the best.
www.plinth-design.com -If you look at this site - look down to the item called Cera-Disc - that is the product I helped develop.
I was testing these against stillpoints ultra mini at £400 odd that I owned. I sold the still points and have many sets of the Cera Disc. Similar benefit much lower price.
I recommended these to someone recently who loves them - his system is excellent.
Few interesting things to consider - you will want a few sets because the benefit is compounded.
For example using them under one item in the system might make the system sound too forward, then using them under another will rebalance it - its crazy how that happens but it does / can. Its happened for me more than once
You have some element of sound control with them - they come with rubber orings that you can either use or not use.
Using them makes the sound warmer taking them off makes the sound more dynamic but could potentially tip poorly balanced systems into harshness - possibly (it did with mine during testing, not so now) so you have options with them.
Lastly you will want them to fit tight under the component you place them under - so you have to move them until each foot grips tight - that is how you get the best performance from them. This takes a little bit of effort and can be a bit awkward depending on what you use them under but its worth the effort. I use a ruler or similar to move them slightly until they tighten up.