My experience with a "VibraPlane" isolation device was that it not only isolated a turntable from the sub-sonic problems mentioned by the OP, but that even the placement of other equipment, like a CD player or DAC produces a surprising improvement in clarity and focus. I understand that they are used with electron microscopes to improve the clarity of the images. There seem to be quite a few used ones for sale on EBay from about $400 to $4000. It works, but they are HEAVY !!
How to isolate turntable from footstep shake or vibration
Even while the Oracle turnable that I use has a built-in springs suspension by design there is a low or even sub-low frequency boom every time someone walks in a room. This becomes really bad with the subwoofer’s volume set high as the low frequency footsteps make straight to subwoofer where they are amplified shaking everything around. It seems the cartridge is picking up the footsteps very efficiently as even a lightest foot down becomes audioable. What can be done to attempt to isolate the turntable from the low frequency vibrations? Interesting, that the lower the volume of the subwoofer, the less the footstep shake is evident and with the subwoofer turned off it is a barely a problem at all.