I have the same setup as you and added an amp and sub to my system for much cheaper than the "pre-packaged" systems. I used this DIY page:

http://www.xoutpost.com/articles/x5/s...ctory-amp.html

to get the info for wiring the amp and then I just made my own sub box. First one I made fit into the right side storage area, but it only housed an 8" speaker. Then I installed a 10" right into the door of the same storage area, using the storage area itself as the box, but I have not been happy with it recently. I plan on just building a box for the 10" once I get the X back from the body shop.

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2001 X5 3.0i (PP/WP/AP) Traded at 213K miles
RARE White on Black - Why does everyone get Tan??
First E53 with 215s on XOutpost!!