There is a way to “hear” the potential in different parts of your system. The method for checking audible noise is to take an amplifier and some magnetic field picking sensor. You can use a coil connected to a microphone amplifier. Or you can take an old cassette player, remove the magnetic pickup that reads the tape, put it on the end of a long stick.
Now just use headphones to listen to amount of noise on different parts of your audio system wiring. What should happen is that you will hear the stronger electromagnetic field at certain points, usually where the biggest problems are because more the current flows more magnetic field it generates. Often you hear also other noise sources like magnetic field mains transformers, so be careful to analyze when the noise comes from wiring that is part of ground loop or some other source not related to ground loop. The “hear” method is worth to try as an additional tool in trying to solve ground loop issues.