Granite surface plate

Surface plates when purchased from machine equipment suppliers are inexpensive. As other have noted, putting a heavy surface plate on the top shelf could cause a pendulum effect. You do have an option of counter balancing the pendulum effect by placing additional surface plate on the lower shelves.

How is your floor suspended?

Audio blazer, don't forget that HRS uses a 0.75" granite platform set in a aluminum shell and sitting on a polymer bed and supported by load specific feet. The surface plate being discussed is 4 times the mass of the HRS.

Granite is useful for isolation, but IMHO, granite needs to be damped to prevent ringing etc.