How to isolate turntable from footstep shake or vibration

Chakster, after I added a Velodyne 15” sub, my Altecs sounded much crisper at the low end and in the midrange, with improved stage imaging. 

FWIW I have a VPI TNT sitting on 4 hand balls, per the last of the VPI TNT suspension designs. It sits in a sandbox on a table, itself on a suspended floor. When I start hearing sound of walking on the floor, I know it’s time to change the handballs, about every 6 years. With new ones it’s quiet. I have a subwoofer that causes no problems whatever--and no high pass filter.

Since you probably have a different set-up, why am I relating this. Because it works so well that if I had a different TT I would get 3 or 4 flat pieces of wood, drill them out so that handballs could sit on them comfortably. Think Ginko.. And sit my TT right on them. Only other thing to do is to shim for level.  If the plinth didn't have sufficient weight, I would place it on a heavy cutting board and place the board on the balls.

Thick slab of Sorbothane foam sandwiched between two marble slabs

@atmasphere and @lewm , I can tap on the 6922's in an ARC PH3SE and hear absolutely nothing with the volume turned up. I'm sure if I tap hard enough I could get an audible result. But tapping on the tube would have a lot more energy then you would get with air bourn vibration. I use super quiet tubes. My instinct is if a unit is sensitive to tube microphonics perhaps you might consider another unit. My only current experience in regards to tubes is my phono stage. So it is limited. But, having to spend money on effective vibration elimination would bother me. I would prefer spending the money on a better phono stage that was not so effected. 
The cabinet my system is in is also extremely rigid, more so than anything you can purchase. My system is immensely powerful using a total of 4000 watts, two 8 foot ESLs and four 12" subwoofers (soon to be 8) and the only device I feel the need to isolate is the turntable which comes so equipped. As I previously mentioned I can put the phono stage and turntable right on top of a subwoofer with no ill effect I can hear. You never want to trust what anybody "hears" and I certainly can not say this applies to every phono stage or preamp as I have only tested one. 
However, If lewm and atmasphere insist there are electronics so effected I am sure this is true. But, If the military can make electronics that are immune to vibration so can the high end audio industry. As far as I can tell none of the electronics I have excepting the turntable are sensitive to air bourn vibration. In the cabinet they are in structural vibration is not an issue. Maybe people should spend the money on a solid cabinet instead of isolation devices. 
@melm , I never have to change my balls. Perhaps you need a turntable that has no balls, something like one of those Audio Technica direct drive tables:-)

I found the easiest fix for my customers was a wall mounted shelf (like the one from Project) on a load bearing wall. Worked every time. Hopefully there’s such a wall behind where you have your equipment set up!