Turntable isolation

I know some people with HW-19's took out the springs and used sorbothane/rubber in its place

I made a video some times ago.https://www.youtube.com/watch?v=mcoBxztdA1sFrom 0:40sec. you can see some measurements with Sylomer.
Sorbothane is not long-therm stable.

One possible issue with any turntable springs is that they aren’t the correct spring rate to be able to handle the extremely low frequency of the seismic vibration. It’s neceassary to get the total Fr down to say, 2 or 3 Hz, ideally, before very good performance kicks in. Alternatively, you can load the existing springs with a heavy slab of granite to reduce Fr.

You might try what was used to good effect in the final iteration of the TNT turntables, that is, squash balls. At each corner a properly sized piece of wood with a central indent on top (such as you can make with a large counter-sink, a spade bit or a forstner) can hold the ball in place with the TT on top of it. Adding something heavy underneath can complete the base: a sandbox, a thick butcher block, a piece of granite or marble.