If you're using the unit in a car, the output's ground will be near 0v - not quite, due to the fact that it varies around the car, but close. If you want to use the 4v outputs with a home amp, use the grounding cable and short the ground lead to the empeg case, which should correspond to mains earth (as the PSU is earthed).

The o/p's are quasi differential as they aren't actual differential drive, just single ended, but with reference to a ground sense. If this ground sense floats then things can get wierd, as you heard!

Hugo