Hmm, I think I found what I was looking for in the original empeg user guide:

Quote:
When setting up the amplifier the gain should be turned to minimum (least sensitive) and then, with the empeg car set to maximum output (0dB attenuation), turned up until your maximum listening level is reached. Too high a sensitivity setting will cause both distortion at high volumes and interference at low levels.
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Mk2a RioCar 120Gb - now sold to the owner of my old car
Rio Karma - now on ebay...