So......what do you use under your speakers?

I find that one has to experiment to determine what is best. Usually, with concrete floors, it is best to couple the speaker to the floor (sharp-pointed cones), but, isolation could, in some circumstances, be better sounding. If you don't like the cones you tried, then perhaps isolation, instead of coupling, should at least be tried.

I use isolation with my speakers and I do this with a Symposium Svelte shelf under my speaker. The shelf has a soft core between two aluminum sheets so that vibrational energy is dissipated as heat from the molecules in the soft core rubbing together. This results in a tighter sound, particularly in the bass. I also use shims under the Svelte Shelf to change the backwards tilt of the speaker.

If you don't like the height of the speaker, it is often possible to achieve some of what you want by changing the tilt of the speaker instead of raising it up. If you tilt the speaker back (top leaning away from you when you stand in front) that often acts in the same way as raising the speaker. This changes where the drivers are pointed and can change frequency balance, and for reasons I can't explain, such backward tilt tends to raise the height of the image.