A pre-amp, mixer or receiver if you are using it's line level outputs to active speakers or subs.
I guess an audio interface could be both a source ans a control unit.
It's common for control units to output a DC pulse when turned off. And control units often turn off quicker than amps and subs.
The pros have turn on and turn off sequence rules.
Turn ON sequence :
1) audio source units
2) audio control units
3) amps (and active speakers)
Turn OFF sequence ...
Cable pickup of Radio frequency interference is a very situation specific type of problem. You need:
a) a source of the interference.
b) a cable of a length and placement to act as antenna.
c) an audio component susceptible to interference at that frequency.
Many if not most interference...
If you are good at math and have a DMM with good frequency response.
Make a series circuit of a resistor and the coil.
Selectric a frequency and a resistor value so the the voltage across the resistor and the coil is the same.
Then do the math.
The Radio Frequency Characteristic Impedance of a cable or connector only matters when:
The length of the cable or connector is a significant fraction of a wavelength at the frequency in question.
So at 50 megahertz that length might be in the neighborhood of one meter.
In the US, 3 phase AC is industrial & large commercial thing.
Residential and light commercial use a single phase, center tapped 240/120 Volt system.
The Safety Ground is attached to the Neutral at the main breaker panel.
The exterior attachment to Planet Earth has little to do with day-to-day...
In the all analog world, in theory the pest plan was to have all the stages go into clipping at about the same level.
In the Digital to Analog world things are more complicated. Just how a DAC handles volume control is not always obvious
It's hard to test for interference suppression, you need:
a) an interference source.
b) cable placement and length to act as a receiving antenna of that interference source.
c) an audio component that is susceptible to interference at that frequency.