I believe in trial and error (experimentation in other words to find what sounds best to your ears). With that said I have found Isoacoustic pucks under your equipment, and possibly a lesser expensive rubber cork rubber squares under the granite shelves should your rack allow you to try that.
Isoacoustic pucks under equipment or platform
I have some thick granite slabs I want to use to help isolate my equipment, my question for best results Do I put the pucks under the granite and then set the electronics on top of the granite or set the pucks between the electronics and the granite?