This combo that works for me out of the the various that I've tried....
a) concrete pavers on wooden floor (>50mm if possible ; stick felt etc so they don't mark the floor)
b) followed by the thickest IKEA bamboo chopping board (20mm) on top of the pavers. The speaker spikes go onto the bamboo board.
tried granite but found they give a harder sound (treble) in my system.
Found concrete pacers to provide better sounding bass (more solid and deeper) and treble (clearer, less ringing).
found addition bamboo on top of concrete paver to "quiten" the sound making various note sound much clearer. Bass also had more texture/detail.
YMMV. Worth experimenting with various bits so see what works for your system.