Joe - what I meant is first to eliminate where you get the surge from - because that's what triggers your speakers to pop I have to assume (and anybody who's an audio guru - please chime in). So if you disconnect the RPM signal - does it still pop? 2nd option is your head unit? if you turn it off - then you obviously don't have an input signal to the HSE - so it shouldn't pop but that would be my 2nd step - first with and then without RPM signal attached. Based on what you said - the HSE is still working w/o issues once the boat runs - correct?
So simply put - you could just turn head unit off and won't encounter anything and after engine is running - turn head unit on and be all good to go as you avoid the surge going thru the system when you crank the engine. Obviously not the perfect choice but that would eventually tell you where the problem comes from. At this point I won't be able to help you with the details but theoretically I see the following options: Fix the problem of the surge - whatever causes it or put a "filter" in the line (either head unit or RPM) so it won't reach it. Again an audio guru can tell you exactly what I'm referring too - I"m only an accountant