These guys make X5-specific speaker kits:

BMW Audio, BMW iPhone, BMW Speakers, Subwoofers, and more | BSW, BAVSOUND, Bavarian Soundwerks.

ETA: I have no experience or affiliation with these guys. Just have been drooling for a speaker upgrade (especially a better subwoofer) one of these days. And these guys make direct-fit stuff.

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2006 X5 4.4i
Sport package, summer package, cold weather package, DSP + Nav
Bought 2/14 with 73k miles