How to isolated stand from springy floors?

Usually bad tubes are more sensitive to mechanical disturbance.

Foot fall vibration on a sprung floor is tough to eliminate. Use a wall shelf rack attached to a load bearing wall will help isolate your equipment off the floor and away from foot fall vibration.

Regarding tube damper rings on small tubes found in preamps, I have noticed tighter bass and reduction in microphonics from airbourne sound waves.

http://herbiesaudiolab.net/rx.htm

More mass. Add a few hundred pounds of mass around under the rack. Bricks will do. Get heavy ones. Concrete like.

If you have access to the basement and it is un finished, you could put in some "lally columns" see link
http://www.lowes.com/pd_210155-46417-P+M204_0__?productId=3201387
I did this when I had an LP12 to reduce the springy response to footfall. Locate the two joists that your equipment rack is supported by and tighten the lally columns to just touch the joist. Then the solid cement floor is transferred through the columns and acts like a foundation to support the floor.
Caution though, if you have expansive soil in your area this will not work.
Another partial solution if you have access is to add "bridging" between the joists. This is a board that fills that gap in a perpendicular position to reduce the flexing that takes place when a joist deflects from load.

As a final note, and not to argue with Elizebeth, but I think adding weight will just prove to further damage to the floor and cause more issues down the road. But that is just my opinion and I am not an engineer.