You should be able to find a non-DSP X5 amp on eBay for under $50. I just did a quick search, and I did not see any at the moment. However, they used to be out there all the time.

If you end up having to replace the amp with an aftermarket unit, I would do it in combination with the speakers. The reason is that you would need to get aftermarket crossovers for the speakers, and for the cost, you might as well do the whole thing.

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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