How to isolate turntable from footstep shake or vibration

antigrunge2, Pretty sure you agree with me completely: Listen and learn. The point I was making is not that any old thing will make a fine arm board. The point is anyone can try these things very simply and quickly - listen and learn - as long as they set aside the urge to make it look good. 

Many times guys agonize over how to do something, spend hours debating this and that, when in far less time they could just go and do it and find out. Listen and learn. 

This is by the way very similar to what professionals do in designing or improving a component. They build a breadboard, basically just a board with room for all the parts. This makes it real easy to swap out parts and compare. They can even compare things like the same part in a different place, or oriented differently, all kinds of things. Only once they figure this out then they package it nice and neat. They listen and learn. Only at the very end do they make it look good. Same thing. 

The thread title suggests footfall issues while using a spring suspended subchassis type of turntable. But the text of the opening post suggests an issue with a subwoofer which makes a loud "boom" that seems to be triggered by footsteps.
This to me suggests a combination of issues. But the "boom" noise coming out of your subwoofer suggests, to me, acoustic feedback. Is it possible that the turntable and subwoofer are in close proximity to one another? If so the simplest solution might be to move either the turntable or the subwoofer further apart. Or aim the subwoofer differently just to see if this helps to eliminate the "boom"...
Otherwise the issue of footfall and spring suspension turntables has been covered within this thread and many others more or less completely over the past 60 or 70 years ever since the AR-XA turntable was introduced in 1961..

I do have some experience dealing with spring suspended turntables, having owned a few Thorens models including the TD150, TD160, TD126, etc. And those will respond well to being parked on a wall rack. The idea being to decouple the turntable from the floor so that the seismic vibration of a footfall on a light flooring system does not reach it. Or there is always the idea of putting your turntable up on a true lab quality isolation shelf such as a Minus-K, or Vibraplane or Herzan. Any of these will decouple the turntable from the floor....for a price. But the wall mounted shelf would be the least expensive way.

And this would, no doubt, be of benefit even to non suspended turntable designs.

Still, based on the opening post I’m leaning toward this being a problem related to acoustic feedback which can be alleviated by moving either the turntable or the subwoofer.
-Steve

https://www.theanalogdept.com/what’s_under_your_table.htm

There is also another condition that might be the cause of the Sub Woofers exuberance.
The Condition if present will be quite easy to resolve, as it simply means checking the Earth Connections are at the correct termination points and ensuring they are clamped effectively.
I refer to this as it has happened to myself on occasion over many years, and very recently as well, when I working with a Selection of Phon's SUT's and Head Amp's 

An Incorrectly Earthed Vinyl chain can produce effects that manifest through the Bass of the Speakers.
When I am trialling with Phon's and Ancillaries, I have at times got the Earth connections incorrect at the time of exchanging.
An incorrect Earth even though not being a Audible Hum, can react to activity around it and produce noticeable Bass Cone Movement.   

Being a perfectionist does not mean you are paralyzed.

<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>

It is my biggest obstacle, GETTING started. I think and think and think.

Then pull the trigger. A guy taught me 50% of getting the job done is just starting.. He was right..

The older I get it's closer to 100%. :-)

Townshends, yes. I have a Seismic Isolation platform and it is quite effective.