you want that rack perfectly level, now, and forever, no matter what is placed where within, so I would not put anything that compresses below the main floor contact.
I bolted new end hardwood legs onto my rack, to get the fulcrum points in front of and behind the weight, tapered because the floor is slanted, shims for final leveling, I could have made them taller as you need.

It is hard to see, but each glass shelf is isolated from the bentwood frame below it on hard rubber spacers
the JVC Victor Plinth’s feet are isolators and adjustable for leveling it, however my wood floor is flexible, my final solution was rubber/cork below the TT’s feet, after they worked, I wrapped them in black tape.

there is a lot of weight, but because the rack is level, it doesn’t move like it used to.
rather than a wide rack, I like separately movable units, my preamp and amp unit has felt feet and slides easily enough, R2R tape is on a separate movable unit left end

and I made a stand/surround so I can move the bottom preamp without moving the heavier amp above

wide open back for access and ventilation
