@jc4659 - I have exactly the same setup in my basement (carpet on concrete), so I took a slightly different approach to others, which appears (to me) to work very well
Under each speaker
- I use two 18" square granite tiles on top of each other with a layer of thin foam drawer liner between them
- this acts as an isolating sandwich
- at each corner of the lower tile I glue one of these
- The speaker sits on top ot the two tiles using its spiked feet
- This larger "base" is just like installing outriggers to the speakers, which minimizes any movement
- after about two months the underlay is compressed under each spiked foot resulting in a very firm platform on which to place the speaker
My speaker weighs about 60 lbs, which contributes even more to their stability
So now my speakrs are
- nicley seperated/isolated from the concrete under the carpet
- very stable (actually rock solid)
- sounding amazingly detailed with a large and spacious image
One other approach to achieve additional speaker stability is to add heavy weight on top of the speaker - this prevents the woofer cone from moving the speaker cabinet
- A friend places a 15lb bar-bell on top of each speaker
- doesn't look elegant - but it works ! 🤪
Hope that helps - Steve