I would first make certain that the legs of your rack are sitting on top of your floor joists opposed to on the plywood between the joists. This will give you the most stability. You don't want to use anything that isn't rigid under the table. I use granite but it's not really inexpensive. You might want to try a large paver block 18" x 18" or something in that nature.
What is under your TT, cheap islolation?
I am looking for a way to isolate my turntable without using a shelf bolted to the wall. I am trying to save money while I re-configure the room. For now my turntable is on top of my stand but you have to tip toe around the room. I am wondering if a big rock iso platform (or DYI version) or some iso nodes under a butcher block would help with the bouncing turntable? I have two tables I am testing at the moment, one is a Thorens TD-160 & the other is a VPI HW19 MKII, both of them suspended. The floors are pretty sturdy, it is a dedicated room over my garage with serious supports built in to the floor.