(And it's not all superstition, and it's not all for tube amps -- it's all in the SNRR of your power stages for every piece of equipment in your sound chain.)
https://en.wikipedia.org/wiki/Power_supply_rejection_ratio
(And it's not all superstition, and it's not all for tube amps -- it's all in the SNRR of your power stages for every piece of equipment in your sound chain.)