The radio communicates with the amp over the car's I-Bus. So, you could simply buy a used amp and connect it to power, ground, and I-Bus. Then, the radio would detect the presence of the stock amp and do whatever it does in response. This would be a good experiment, to see if the volume changes in the car when the radio "sees" the amp.

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2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input