Very odd behavior. When I hear of something like this I think of a bad ground, missing ground, incorrect ground, a ground, ground, ground problem somewhere. It can be hard to track down, especially when the error is often illogical to begin with. Physically check even those terminations that visually appear to be secure.
Next, I think of EVERYTHING audio should go to the 'C' post of the Perko switch and not battery direct, including the helm buss and starter/alternator feed.
Next, I think of bad voltage regulation where the alternator is developing voltage that's too high. This can easily be measured.
Next, I think of a bad audio component that must be eliminated or substituted to isolate.
Time to roll up your sleeves.