A Copernican View of the Turntable System

T_bone,
Is your concrete floor 'suspended' or cast on the ground?
A suspended slab can often pose problems due to dynamic movement and deflections.
It's true that seismic movement is hard to protect against but most other external influences (even in big cities) can be minimized especially by wall-mounting.
The de-coupling effect of metal brackets combined with wood-based shelving cantilevered off masonry walls works in all but the most complex of situations.
SOUND

Suspended. I don't worry about whether my floor will flex though. I know it will. Earthquakes affect the sound because the building shakes (for those not used to them, earthquakes can be 'noisy'). Having had a setup on the ground floor (concrete slab) before, I know it will move too. That's life in the big city...

Unfortunately for you, seismic design entails making very flexible structures......especially at the joints.....those between column and beam, load-bearing wall and floor.
They are designed to move significantly......virtually like a hinge.
Structure-borne feedback in these conditions is almost a given.
I now understand your problems :^(
Air-borne feedback is very rarely a problem despite what you believe.

I am happy to have the seismic protection for when earthquakes come along. But the resonant frequency of those is quite low :^)

However, I notice the difference between having an isolation platform and not having one. And I notice the difference in loading one differently. And I also notice acoustic feedback from time to time. Wish it weren't so, but I do... But that's just me and if noone else has a problem with sound waves in their listening space, I guess I'm just unlucky... :^(

The result of the structural design for seismic loading creates a building (particularly a high-rise steel or concrete structure) which is continually 'moving' even when there are no quakes.
We are not talking resonant frequency here......we are talking constantly moving structures which are setting up wave patterns of frequencies from the subsonic possibly even into the low twenties.
It's not a good look.....but there are solutions which however would require specialist knowledge.
If I were you....I'd try my Micro SX 8000 or any other heavy unsuspended deck and see if there is an improvement?
I'm betting there is:^)