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Of course it is not drawing the power through the head unit, but the head unit is sat on a circuit that can only draw so much current.
The Amp running just speakers can also use this same 12v feed, however if you have added subs you need to take a direct 12v feed from the battery.
It is worth doing this anyway as running the power cable down the side of modern cars with can bus can cause interferance as the power cable picks up nopise from the data network.
The battery in the boot of a BMW makes it a dead simple task.
Now, looking at your settings theyt are incorrect.
You need to set the front and rear levels to 8v, and do that NOW!
Turn is as far as it will go anticlockwise.
You also want to set the pass on the front channels up properly, unplug the sub speakers from the amp, now play something with bass, set it to 200hz or whatever it goes up to (clockwise) and slowly move it back towards 20hz. Keep going until you do not hear it going any deeper, will be about 100hz (approx).
Fine tune till you are sure it is as deep as it will go, you want as much bass from the fronts as possible, but you also want to stop bass you can't hear from reaching the drivers so you can push them with no distortion.
Now you want to set the subs up so they match what you have on teh fronts, so if you got down to 100hz on the front speakers set the subs to do everything below 100hz.
Now when this is done you fine tune the sub gains, the drivers are not as sensitive so you will have this above 8v, probably around 6v, it may take a few days of tweaking to get it so you can hear them but don't know they are there. If that makes sense?
Then use your bass and treble to fine tune.
With the rear channels I actually use the low pass filter, so I cut out the treble so it gives some rear fill but doesn't drag the soundstage into the rear, vocals stay up on the dash. So I set it to cut out everything above 5000hz I think it is, and then use the high pass so no bass below 100hz gets through either, so they are only there to add some fill in the rear.
With this set up you should be able to drive the speakers very loud, no distortion at all and without risking damage to your head unit or amp.
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