The sound is distorted at lower volumes; as soon as I turn the volume up beyond a particular threshold, the rumble goes away. It doesn't matter if the volume is adjusted using the knob on the radio or buttons on the steering wheel. Also, the automatic volume adjustment due to speed kicks in; so if I'm driving slowly and the volume is just one notch below the distortion threshold and I speed up, the automatic volume increase causes the volume level to rise above the distortion level. I've tried replacing the amplifier with one I bought on used from the Internet, but had the same problem.

Did you ever figure out what the issue was?

Jim