Not sure if you had figured out your issue, but I just bought an '04 X5 and none of the lower speakers worked.

It ended up being that all 4 speakers were blown. I popped the door panels, cut the wire terminals and tested it on other speakers.

Gofigure, after ordering a DSP amp and thinking the Alpine amp was blown, it was the speakers this entire time.

It's VERY odd that all 4 speakers would blow at the same time. I even jumpered past the connectors and solder joints, to no avail.

Hope this helps someone