How to isolated stand from springy floors?

I concur with Elizabeth, but suggest using dumb-bells instead of bricks since they are relatively easy to find and install. They also provide a more effective way to accumulate mass compared to bricks.

The ones to look for are Hex-Shaped with a Rubberized Covering. 4 50-lbs bells arranged in parallel, with a 1/4 or 1/2 inch Marble Tile placed on top should provide a fairly even surface to put your stand.

If you can access the subfloor, put cross joists or sister some additional joists onto the existing floor joists.

It sounds a lot harder than it actually is.

Like the additional joist idea (sister). Use adhesive as well as nails or screws.
You could also box the joists (every other pair) with 1/2" or 3/4" plywood, again using adhesive and nails or screws.

Either of these choices should stiffen the floor considerably. The adhesive is important. Both ideas requires floor to be open underneath.

From your pic it looks like hardwood, IOW a floating floor, so how are tighting up the joists of the subfloor going to help? There will still be some "spring" in the floor.
Even with mass loading, there will still be some bounce. The floor needs to be damped, with the addition of isolation footers under the component and if possible, isolation under the shelf.

Also IME, do not put any iso feet under the rack; it is likely to affect the performance of your TT. I'll bet your TT is very happy being on a heavy stand touching the floor.

As a quick effort at low cost, what I have done is beneath those components where there is no interposed individual maple board base supporting the component, simply place three or four Cryo'ed Mini Springs between the component and the existing hard, flat base.
Even though some of your components may quiver when you walk by, there won't be a jarring shock to any of the protected components.
The sound you hear from your suspended components will surprise you with the improvement heard.
Long ago, the Aurios ball bearing approach also allowed for this quivering or rocking when footfalls affected components on my equipment stand, but the sound improvement was nowhere near the significant upgrade with the Mini Springs from Machina Dynamica.
There are variations in implementing these springs.
This minimalist approach is fast, simple, audibly impressive -- even though not the "ultimate" implementation of what is termed Baby Promethean Mini Isolators by Machina Dynamica.