just don't use a GFI breaker electrically they're very noisy.
The quietest mains connection can be had by using a subpanel with an Edison fuse, vs. a circuit breaker.
The switching transients from the water heater may possibly be adequately isolated by running the audio rig on a DEDICATED CIRCUIT. If that doesn't completely stop the noise then you'll need Audio Prism Quiet Line, or some other transorb such an a GE V130LA20B M.O.V. installed across the line. Many line conditioners incorporate MOV's integral to the design.
The quietest mains connection can be had by using a subpanel with an Edison fuse, vs. a circuit breaker.
The switching transients from the water heater may possibly be adequately isolated by running the audio rig on a DEDICATED CIRCUIT. If that doesn't completely stop the noise then you'll need Audio Prism Quiet Line, or some other transorb such an a GE V130LA20B M.O.V. installed across the line. Many line conditioners incorporate MOV's integral to the design.