The amp and other components will usually benefit substantially from isolating the rack as a whole. Still, it's always most essential to isolate/decouple each component individually. Electronic components self-generate micro-vibrations, transformer hum, motor vibrations, etc. With components each well isolated, isolating the rack provides additional, supplementary benefit. As far as priorities, I suggest addressing the rack after isolating the components and speakers.
Steve
Herbie's Audio Lab