Twl gives good advice. I solved a problem with floor-borne vibrations when I had my rega 3 on a sideboard by getting a slab of 3 inch thick firm foam, placing a 2 inch thick concrete slab onto the foam (compressed the foam to about 1.5 inches) and then putting the turntable on the concrete. This completely cured the floor vibrations from feeding back to the turntable and got rid of the rumble.
I would say that floor vibrations are only a problem if your room has a suspended floor.
I would say that floor vibrations are only a problem if your room has a suspended floor.