Assuming that your amplifier gains are correctly set (i.e., your maximum "normal listening" volume of the empeg is 0db max), then you should be able to have essentially no noise. Or at least, so little noise that you can't hear it at all when the car is running.

If it's too loud for your speakers when your empeg is set to -20db, then that's the problem: Your amps are turned up too loud.

Also, is the unit a Mk1 or a Mk2? If it's a Mk1, then you're probably going to get some noise. Sorry, forgot to ask that question at the beginning.