Thanks for the replies. I just picked up my new to me X5 the other day. I look forward to upgrading the audio. I may have to sell those CDT's. My previous setup got complex and I would like to simplify. Simply switching out the speakers would be a good start! I have noticed some crackling at high volume.

Are there any issues with connecting 4 ohm speakers to the stock amp?

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2004 BMW X5 3.0i - Mine
2011 BMW 328i xDrive - Hers
2002 BMW 325xi - waiting for a new owner