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The stock Bose™ Concert head unit was kept in the car for a number of reasons, including the steering wheel button integration, and dashboard display information. Although there are a number of other units which may outperform the Concert, I was not prepared to do the work to integrate a third party head unit as seamlessly as the Bose.
The head unit sends line-level signals in an unshielded wiring harness to the Bose amplifier that is attached to the subwoofer in the right rear of the allroad, behind the wheel well.
The amplifier receives the line-level signals on 4 pins of a 25-pin connector, with a 5th used for ground. The amplifier appears to apply an equalization curve to the signal to compensate for nonlinearity in the frequency response of the drivers. After the signal is amplified, the (presumably) high-pass speaker-level signals pass back through 8 pins on the 25-pin connector, while the (presumably) low-pass speaker-level signal is routed to the subwoofer via a separate connector.
The door speakers consist of a 6.5" paper cone woofer with foam surround and 1" cloth tweeter wired in parallel with no crossover (!)
The subwoofer consists of a 6.0" paper cone woofer with foam surround in a 4th order vented box designed to receive the amplifier, and fit in the well behind the right rear wheel.
My plan was to use the cabling from the head unit to the 25-pin connector, where I would connect a custom wiring harness with shielded line-level RCA cables and speaker cables.
This wiring harness would connect to a multi-channel amplifier with integral crossover.
This amplifier would in turn drive new 2-way speakers mounted in the factory door locations with a series of passive frequency dividing networks.