These feet list an ideal weight amount which is really important. You can use any damping pad or foam or whatever product but if the durometer/stiffness doesn't match the weight of your amplifier, then it won't work very well.
If you can, try to use a product that lists an ideal weight or a range of weights per isolation foot. Divide the amplifier's weight to 1/4 and that's how much weight each foot should be supporting.
This exactly!
Here's another place to look:
https://www.mcmaster.com/isolation-m.../height~1-2-1/