How far is the CP4 from the Marantz and APpro?? If they are on a different electrical subpanel you can get surges even though the ground too.? My policy is if an RS232 device is on a different electrical subpanel, different building, or to something outside, I surge protect the RS232 port.? In most cases a simple opto isolator is sufficient.? However, if it is going to a pool or irrigation system, those surges can easily exceed the 2KV rating of opto-isolators. In that case you will need to use fiber or going wireless.
The thing this is strange in your case is, at least for the case of the Marantz, it is just and RX and TX line that normally idles at -5 to -12V.? So disconnecting them does not change the logic level.,? Hence the reason it sounds like a latch up issue.
Why don't you monitor the RX and TX lines when the failure occurs and see what side stops communicating.? Also measure the difference between the grounds when disconnected,. It is not atypical to be 60VAC between the grounds of some subsystems if they are not grounded correctly.
Good luck,
Steve
