Any hard and rigid material will store unwanted mechanical energy (vibration) and release it over time as extra energy into the chassis of the component. This will contaminate the signal flowing through the component and make it impossible to faithfully reproduce the original instrument as it has been captured in the recording. Other materials that resonate (wood, acrylic, etc.) are also not appropriate for use as a vibration control device as they too will impart their own sonic signature on the signal.
Another issue with using any rigid coupling (a slab of rigid material, a rigid foot, etc.) is that the component will then be in direct connection with all of the unwanted vibration in the floor trying to enter into the bottom of the component. It will also allow any sympathetic vibration of the rack and shelf support (sourced from air-borne vibration) a direct path into the component.
Decoupling the component from floor-borne vibration and providing a method to convert the unwanted mechanical energy into benign thermal energy is the most effective method to address ALL of the sources of unwanted vibration.
Disclaimer: I am a manufacturer of vibration control products.