Having trouble reducing the vibrations on my turntable setup

The rationale for using only three footers under any single piece of gear is that three points define a plane. Whereas when you add a fourth point you don’t necessarily have a plane. Hence the device can rock back-and-forth on any two of its 4 points. Or, one of 4 is not bearing its share of the mass which shifts a greater than average burden to another bearing. I agree that mounting a 100 pound turntable on three points is a bit scary, but it does give confidence that the turntable is nesting on a plane. After that it’s relatively easy to level the device by adjusting the height of one footer. 

I agree with whoever said that banging on the plinth or the shelf as a test for isolation  is beside the point, at best.

I think minus K, vibraplane, and Herzan are kissing cousins, business-wise. If you don’t mind an ugly industrial look, herzan platforms with appropriate load capacity can sometimes be found on eBay. For much fewer $$$.

MC, Nodes for vibration should not move around.  However, the number of them might go up or down depending upon frequency.

DO Nuclear subs and CAT machines use SRA isolation devices? I’ve been around a lot of electron microscopes, and they don’t.

Any shelf will vibrate at a certain frequency.  Depending upon that frequency, there will be regularly spaced nodes that remain stationary even while the shelf is energized and vibrating.  If you can locate those stationary nodes and place the points of your cones directly on those areas, then the cones can provide isolation.  But it's very tricky to locate those nodes.  Use a stethoscope.  Or I suppose it is easier to choose another type of footer.