generally, you want every level to be as low as possible.
Start by setting everything to it's lowest setting,
turn all the amps, the volume and loudness settings all the way down.

Now turn up the volume as high as you can without getting distortion.
You should get pretty close to the top.
Then turn up your loudness setting up as loud as you can without distortion.
lastly turn up the amps.
The idea is to provide a nice strong signal to the amps, then have the amps do as little amplifying as possible.

the other plus to this method is that if you'll be able to turn up the volume all the way with no distortion, you'll never hear distortion, (unless of course it's encoded into the file)
because no one in your car is going to be playing with your amp gains.

Where as if the amps are juiced up and you can only turn the volume halfway up before you hear distortion, one day, you'll be adjusting the volume, hit a bump in the raod and send the voulme into overdive by accident.
Then you'll be turning into the car stereo store to buy replacements for the speakers you just blew, not to mention your ears and wallet.

LOL