Hmm, I'll take a stab at the obvious so the smarter people around here can try the more esoteric stuff:

Is it hooked up via aux? Is the volume setting on the 2nd empeg normalized at 0 db and locked if so? Any weird equalizer settings? If you use Hijack to force AC mode while in the car does the problem still exist?

That's it, I think I'm out of obvious stuff...

-- Gary F.