When you use the AC adapter (where you say the noise goes away), is the AC Adapter plugging into the AC Adapter power jack on the empeg, and are you plugging it into a wall socket to get its power in that case?

And when you use the 12v cig lighter source, is it connecting to the car dock continous power wire (which IIRC is yellow)?

I wonder what would happen if you got a 2.5mm connector and wired the power source from the 12v cig lighter powere into the AC Adapter jack. In other words: Same power source, but, different jack on the player. Wonder if the noise would go away then or if the noise stays.

My theory is that the noise occurs because of a ground loop between your portable amp and the empeg when they are both powered and grounded from the same spot. The reason it goes away when you use the AC adapter is that it's powered and grounded from a completely different place than the audio amp in that case.