Okay I have an E70 with the base audio system.
I do know there is no external amplifier. It is built into the head unit. Only the higher end systems have an external amplifier.
I am going to modify my system. I have es100k focals for the front with separate tweeters, to go in the factory locations. I have a focal compact 4x55watt amp to go in. 2 channels for the front 2 channels for underseat woofers. This will be installed in the dash, thinking behind the glove box. Will initially be using high levels into the amplifier. Then will code the car to hifi to get rca outputs, low level. But more importantly remove DSP, so will be a more flat signal.
I have not removed my door trims yet to expose the wiring. So who knows what type of wiring I'll find. But I think it will be single wires in a loom so to speak. I'll chase these back from the doors to the head unit and under floor 6" mids. Pretty sure these have capacitors fitted to filter the high frequencys out.
If you want the best sound you need to code to get rid of the DSP, or fit an amplifier that has the ability to modify.
I know this does not exactly answer your questions

Chris

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