Haven't read entire previous 27 posts, so pardon if I repeat, but I'll give it a shot, WTH!!
I'll head down the "conducted" path first(as opposed to "radiated") for just IMO, it is the quickest/cheapest directest test to possibly deduce a few things, including ruling out/in it being radiated and/or conducted.
Again, pardon if I repeat something/someone, but as a test.......Have/can you hand carry a single battery(or isolate an existing one) into the boat, sit it down beside the head/amp(any/all of the audio components)and do some quick temporally wiring(alligator clips and such) in such a way as to remove and wire just the audio pieces away from the boats wiring(including frame grounds if applicable) and have only them connect to this ***TOTALLY*** electrically isolated battery?
If so, then re-create/fire-up the situation/component/s that cause the issue/s?
Now what happens when audio system is powered JUST w iso'd battery?
Dead quite? = "Conducted".....usually.
Typically this is noise entering through power lines which can also be exacerbated by poor grounds(not directly wired to the source batt) and/or ground loops.The cause is typically due to poor line filtering(replacing inverter w higher quality could help) in high freq switching inverter's power supplies where switching transients are conducted onto and then of course back out through power transmission wires and are picked up by other devices powered from same wires/interconnections. Typically reduced by line filtering(internal and external) both at source and receivers, single point ground(no daisy chaining) and/or power, signal(you've tried this) isolation.
*Same? = "Radiated".....usually.
Typically induced on poorly/non grounded/floating conductors, chassis and/ or inherently higher impedance (RCA) transmitter/receivers in/outputs and associated inter-connecting transmission wires(i.e. open circuit high impedance inputs as you've discovered) ........The cause is electromagnetic radiation(replacing inverter w higher quality could help).
Reduced by ensuring proper single point source grounding, common grounded metal chassis(boxes) both source and receivers, use twisted shielded pair wiring(both sig. and power) if and where possible to maintain grounded shielding and highest possible common mode rejection ratios are maintained, use high quality coax wire/RCA's interconnects, lower output impedance of source and capacitance between signal source and receiver, use differential output/inputs instrumentation type amp's on the low level/high impedance audio signals(balanced).
*Radiation is induced on conductors, thus becomes conduction, but due to radiation.
***Just the minimum amount(both length and count) of wires for functionality of the amp and head unit going to this battery, i.e., The amp's +/- and the head's +/-***
***Important not to have any other grounds, lighting or standby etc., etc wires from any of the audio systems connected into the boats system***
***Low impedance(old school Simpson type or DIY) ohm meters can be used to ensure there is good isolation between the two grounds and supplies(boat and test battery) if unsure and available. If truly isolated, low impedance ohm meters will bleed off parasitic charge and indicated proper resistance with power applied ***
Just a shot in the dark
