You say that the noise gets louder as you remove RCA connections until you remove the last one, whereafter the noise is elliminated ? Sounds to me like you are pulling current through your RCA connections. Do a couple of checks: Grab a multimeter and with your RCA's disconnected and the system turned on, measure the voltage between 1:your negative battery post and the RCA shield on the Empeg and 2: your negative battery post and the RCA input shield on the amplifier. If your Empeg has a voltage difference between shield and ground (battery), you need to check all your ground connections and your docking connector. If that's all good, then you could have an internal ground problem (warranty/contact support ?). If there is a voltage difference on the amplifier side of the system, the same checks hold true for the amplifier. The third alternative (maybe temporarely) would be to disconnect the shield connections at the amplifier end of your RCA patch cords, leaving just the signal connections (center pin) hooked up. Your line level signal return path will then be forced through your main power connections, elliminating a ground loop, but not solving that one of your pieces is still hunting for a better grab of the negative battery terminal !!
Good Luck

Lars