Make sure you have a good ground from the amplifier to a metal surface in the car. Make sure this surface isn't paint covered and use a solid wire if possible (not stranded) to complete the ground.

Does the Hum pulsate?

Does the Hum get louder or softer when the ignition is turned?

Make sure your speker cables are not coiled or wraped up and or near any transformers, batteries, or other wires that have current running through them.

The most important thing is grounding.