not the least an expert but my trials began with cork.
jbl l890s and l8400p. on carpeted wood floor, third floor. lots of floor resonance naked on floor.
1. rubber/cork pads-somewhat less boom. bass was a tad smeared maybe.
2. rubber/sorbothane-mushed up upper bass, low mids.
3. 3/4" horsestall mat-first positive move. lessened floor resonance enough to be noticed. bass a bit more detailed. mids sweetened a lil. highs dialed down a stitch.
4. horsestall mat, 3/4" hardwood board, naked feet. tighter bass. less resonance. i tried the pads in between the feet and boards here also but it was a step in the wrong direction.
5. horsestall mat, paver, naked feet.
boom. a big drop in floor resonance.
much tighter bass and detail. wasnt expecting the mids to shine more but its sounds great. the highs smoothed some which is ok because the l890s can be taut on some material.
the subs are on iso2000 substands on pavers.
this is where ive stayed for about a year.
is it the best and scientific, nope? but im definitely happy with how it sounds. with the benefit that it might not be as loud to my neighbors now.