I did do a little experiment, and the headphones weren't too bad. I managed to find another cable in my box-o-wires, and tried again.

I managed to discover that there is indeed an audio output level adjustment on the RoadyXT, but cranking it up only made a moderate difference.

I think I am going to try buying a Ground Loop Isolator and see how well it cleans up the signal. The hiss/whine is really bad.

Does your iPod sound OK if you have it plugged into the 12V power jacks? Or is it self-powered when you have used it?