Your first problem is assuming that the "power is dirty". Electricity isn't like a water supply that can get contaminated upstream.

What you're experiencing is most likely a variation on a ground loop, which has nothing to do with whether the power source is dirty or not. Playing the player with the A/C adapter has not absolved any component of responsibility, it simply confirmed you have a ground loop problem.

Have you looked through this FAQ entry and the documents linked therein? I see you've already hit Crutchfield, but there's some other resources there, too.

More details about the system, including details about the amp, and perhaps a wiring diagram, would be useful in helping diagnose the problem.