Audio Systems: hissing sound in amp, compact disc player, rf interference

Question
I installed a Pyle plmra620 amplifier in my boat and cannot get rid of a background hissing sound. The amplifier was hooked direct to the battery with 8 gauge wire for power and ground. The distance is 14 feet. The 4 speakers connected are Marine Infinity 612m 4 ohms, power handling 75 W, peak 225 W. Radio connected Sony fm/am compact disc player cdx-h905ip. Wire used to connect speakers is Monster.
Even circumventing the radio, the hissing sound remains.
Suggestions?

Answer
For a test, make sure you have disconnected the Sony player completely and that the Sony unit power is disconnected from it entirely.  Do you now still have the hiss sound?  

Next, do you know how to short the inputs?  This takes a shorting plug.  Take a blank RCA phono plug that has no connections and solder in a short from center pin to outer shell. Some electronic stores sell them already made.  In any event, if you can short the input and still have the hiss it is either a defective amplifier or an rf interference problem.  Try each channel and see if they are the same for hiss level, etc.  

If the hiss is rock solid and remains in either channel with or without the input shorted it is likely a defective amplifier and should be returned and replaced by the dealer.

Sounds like you have done everything right. Hope this helps you get at the bottom of it all.  Let me know if there is more to the story.