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[Brock]

could your head unit be bad and a loose solder on the board be causing it....if not too much a pain in ass you could remove HU and put a buddies in or the old HU...and see if you get same hissing....other than that I dont now. Does not seem to be volume related...
did not say if itwas same for radio as well as CD music...or I did not remember reading if there was a difference.
If constant.....are amps running parallel or inline? Could wiring harness going back to 400w AC plug in rear be putting off enough juice to cause interference?

Only thing I can think of ....and it is a PIA....is slowly swap out every thing one piece at a time and see if anything stops it....I usually start by swapping wires....with one length of new wire...first one speaker then the next...or amp wires...same thing. Disconnect one amp then the other ...swap their locations... check for loose tiny metal BB or other item that is just barely touching and given right harmonics buzzes...cant help on hisses...LOL. Usually grounds and loose connectors are popping and static.....hissing is outside interference and static.

I wish you luck and damn glad I am not one having to figure this out as it is something that is going to bug you to death until you find it.