The usaspec doesn't seem to have any gain setting that I can access, and the iPod seems to be fine. I might move the wiring to keep the magnet at the bottom of the ipod closer to the unit, see if that helps. On normal (medium volumes) the hiss is pretty much not there.

Just moved the wires, no difference. I can't hear any interference when the music is playing and the engine is on, so I'm done messing with it. The bulk of my problems were sOlved with the new cable.