Sota Sapphire and Isolation?

Do whatever you want. But do realize that the built in spring suspension of the SOTA is there to filter out all the stuff you’re talking about where it counts, before it can disturb the platter/bearing/tonearm/cartridge. I’m not saying it’s perfect, but adding a second independent spring suspension to the closed system can only interfere with its function. This is demonstrable with math and physics, not just a matter of my opinion.

Spring on Spring is not without risk of very high gain low frequency feedback as constructive and destructive resonance will happen… Look for ( failed ) example of double sprung car suspension…. but hey…. 

I’m not saying it’s perfect, but adding a second independent spring suspension to the closed system can only interfere with its function.

@lewm Due respect, but you can’t know that as there are too many variables such as it’s not spring on spring — it’s spring, footer, plinth, then the sprung suspension.  There’s no mathematical or physical model for that combination, and there’s no way of knowing how the resonance frequency of the footer springs would interact with that of the suspension, so again just too many unknowns.  And as you say, the SOTA suspension isn’t perfect so the big/ultimate question is really if significantly minimizing shelf vibrations could have significant benefits that may (or may not) outweigh any negative spring resonance effects and absolutely no way of knowing that without trying.  If it’s me I’d try the spring footers just because no risk/cost to do so then I’d try a thick butcher block that’s cheap as well, and then I’d try something like a Ginkgo Cloud or other absorptive platform and see which combo sounds best.  Always fun to experiment and learn, and often the results can be surprising and counterintuitive.  It’s kinda like implementing room treatments and figuring out the right combination of absorption and diffusion — you can digest all the theory you want, but you’ll never know unless you just try different combinations for your specific situation.  Anyway, at the very least there’s some good food for thought for the OP to chew on here.

I said nothing for or against the Ginkgo Cloud or other solid or semi-solid support systems designed to absorb energy sans springs or soft rubber mounts. (At least, I don’t think the Gingko uses springs; I could be wrong there, and if I am wrong, then I would not use the Gingko under a SOTA tt.) Yes, you DO have a way of knowing without trying (contrary to your sentence above) why spring on spring can be a very bad idea. (It’s never a "good" idea but by chance one might not create serious problems.) Just read up on Simple Harmonic Motion and spring rates, etc. Can you get away with it and using subjective judgement go away thinking there is a benefit? Sure. Happens all the time in this hobby. Consider this thought experiment: you have a nice modern sedan that has a spring and damper suspension conducive to a nice smooth ride down a typical imperfect highway.  Now put your sedan on a flatbed trailer, strap it down to the bed, and ride in your car while someone else drives the trailer down the same highway.  The trailer has its own suspension with its own spring rates and damping. You will be tossed around a lot more in your car tied to the trailer than you were when your own suspension was ameliorating bumps in the road.

In my experiences and one that has not involved spending $0000's on a Sub Support for a TT, there are methods that will tidy the TT's presentation up.

After introducing other TT set Ups to methods selected by myself, there in not a ubiquitous method that works equally in the environments where loaned support methods have been demonstrated and offered up for extended loan periods.

The method adopted that has the most success in relation to my experiences and the one that is today superseding all others used in the past, is to used a Sub Plinth produced from a Phenolic Resin Impregnated Densified Wood, produced with a Cross Grain Structure, with a Thickness of 25mm (1").

The Densified Wood Board depending on weight to be mounted as a TT> DW Boards combined weight has been to use a Pneumatic Footer under/over DW Board, such as a AT 616 footer. Other Brands are available and will do similar as a footer, I have heard them in comparison and know there are valuable similarities.

If TT weight is not too much of a concern, the Solid Tech 'Feet of Silence' with a O Ring suspension has been the most valuable footer used for tidying the overall presentation up.

As a side, I firmly stand by the notion proposed by myself on the odd occasion, that the  Aluminium Base used for mounting the TT's Parts and then Suspended on the Sota, will be substantially upgraded if substituted for a Phenolic Resin Impregnated Wood Board similar to the constriction method already referred to.