Tom, I agree (as others have stated) that if you have a springy floor you are going to need to use a wall rack for your turntable. That wall rack is probably the easiest and least expensive fix. I have a very hard time believing that someone makes a rack that sits on the floor that is going to correct your issue of the turntable.
Is your rack located in a room that has a basement under it? If so, could you add some joist supports under the audio area to reduce the springiness of the floor?
http://cdn1.tmbi.com/TFH/Step-By-Step/FH00APR_BOUNFL_04.JPG
http://www.google.com/search?q=install+support+beam+in+basement&nord=1&biw=2133&bih=1022...
Is your rack located in a room that has a basement under it? If so, could you add some joist supports under the audio area to reduce the springiness of the floor?
http://cdn1.tmbi.com/TFH/Step-By-Step/FH00APR_BOUNFL_04.JPG
http://www.google.com/search?q=install+support+beam+in+basement&nord=1&biw=2133&bih=1022...