Define "Pure DC source"?

Do you mean connected to a 12 volt battery of some kind? Can you offer more details of the power source in that case, more details of how it's wired up, and, how it's connecting to audio output?

Example of something that might cause a ground loop:
- You have the Empeg connected to a battery power source
- You have the RCA cables connected to some kind of audio amplifier such as a powered speaker system
- The powered speaker system is being driven from a power source that is different from the battery to which the Empeg is connected.