Gains too high can be a problem but if the gains are set correctly (using a clean source like a CD, flat eq, HU at 85%, gains up til distortion then back off 1-2 oclock), lowering them may just mask the true problem.
Since engine is off, that eliminates engine noise.
Ensure Power cable is not next to speaker or RCA wires.
Try working backward;
-Unplug the RCA from amps, still hiss? check speaker and power are not together.
- Direct connect amps to mp3 player with 3.5 to rca cord. Make sure mp3 player volume is down. Use different RCA or source player if hiss still exists.
- Hook up RCA cables and use barrel connectors on the HU end of amp RCA's connected to mp3 player
- Run a separate, longer RCA around from HU to amps and test.
- Run a separate speaker wire directly to a speaker
Basically try to isolate each piece to determine where the hiss originates. Cheap, thin rca can be a problem as can an older or pinched connection or 50 other things.
1991 MariStar 240sc, 454, tower, WetSounds, Krypt and Rockford Fosgate