I'll bet the Audiovox can only handle around 1v inputs. The car sled outputs are 4V outputs. 1V levels out of a 4V output means about -6dB down which agrees with what you're seeing (hearing). Not much you can do really. In this case I'd recommend leaving the volume on the empeg at the maximum possible and using the standard head units volume control. This is the best method for noise reduction anyway.

You can lock the volume control but I think only to 0dB - I don't have mine here to try.

As for the volume being very quiet at -10dB - I can't think of why if my explanation/analysis above is correct. Does the modulator have a gain control of any sort?