FWIW, I have a similar situation with my double wide Billy Bags rack. However, instead of spikes, that rack has flat glider feet that have a profile like the products Scotty linked to. When we removed carpet and installed hardwood in the listening room, I attached 1/8" thick self adhesive discs of dense felt to the underside of the glider feet and it works like a charm on hardwood. As long as the receiving pocket for the spike tip in the disc is deep enough to hold the spike in place when you move the rack, you could use those with felt to do the same thing. Home stores like Lowes have many different sizes of the felt discs. If you need to trim them, heavy duty sewing shears do the trick.
If you have access to a drill press, you can modify the aluminum slider feet to have a deeper pocket as well.