Turntable on suspended floor - shelf or braces?

djm,

I should have explained that I used an automotive hydraulic jack to raise each of the three floor joists just enough to slip the carefully measured 4x4 posts into place. Then after lowering the jack I didn't even need to nail the 4x4 posts to the joists, but I did drive a couple of framing nails just in case.

Also heavy duty leveling jacks are available for mobile homes, but the three posts and footers were not expensive.

For an easy fix, cram a few paperback books behind the rack or furniture the 'table sits on (as close to the top of the rack as possible) so that the wall will essentially brace the rack and keep it from moving too much as the floor bounces. The more tightly you can wedge the books in there, the better. Fremer mentioned this somewhere on Analog Planet, and it works.

Ablang.....yes - very effective.

Not sure is this is relevant, but I bought an older but clean Linn table a few years back and stuck on top of my rack where it was NOT happy...then (of course) read the "official" manual which suggested a lower firm table and it's worked perfectly since. Low mass, level, sturdy table...done. Got lucky I suppose.

Sounds like you have the right idea and bolstering up the floor in your case by jacking it up from beneath would be easy to do.
I think a wall shelf would be best unless you have cabinets or something else on that wall that would themselves cause vibrations when accessed. But jacking up the floor beneath that wall will make that wall even more stable. And if you decided against a wall shelf a platform/table close to the wall also minimizes foot induced vibration.
Rega recommends against heavy dense platforms for their turntable design to sit on. I asked about making my own concrete slab for stability and they discouraged that method, saying that it tends to store and then amplify energy/vibrations. Suggesting instead a lighter thinner surface much like the Rega shelves. One that in their words dissipates the energy instead, much like their lightweight turntable designs are intended to do.