Your right you cannot tell what level the headunit its at,
Turn all the gains down on the amp/amps
Turn the headunit all the way up then knock it back a tiny bit, from my experience the BMW headunit is programmed so it does not distort like after market units.
Leaving the headunit at this level increase gain until you hear the speaker starting to distort then knock it back around 10%, once completed unplug those channels and go to the next ones so you can hear them properly.
One other thing to take into account is the crossover settings/filters, depending on your configuration you will need to filter the bass going to the mids & tweeters however if your running them on the same channels as the underseat woofers then this is not really feasible.