Hum is often caused by poor filtering of the AC in the power supply. In the case of the Quickie that is impossible. The power supply has nothing to do with AC.
So it is more likely either induced hum or ground loop/grounding hum. Induced means it sits too close to something radiating a strong AC field. This is easy to check, move it 2 feet from any other piece of equipment. If you can't hear the hum being reduced as you move the Quickie it is not induced hum.
This leaves a bad interconnect or loose grounding wire on the ground reference wiring in the Quickie. Start at the two RCA input jacks. Touch up the ground wires there, the outer connection, not the center one. Follow those wires and touch up all the solder joints that are "grounded." Of course they are not grounded in the Quickie, it doesn't have a power cord that brings in the ground. The ground is in the other components.
