Forget the OEM sub setup, its a POS. Just my opinion.
You could go with the BSW sub package:
http://www.bavariansoundwerks.com/pr.../X5-Subwoofer/
Or you could go with the JL audio stealth box and pick your own amp.
http://www.crutchfield.com/S-JVFCaTw...spx?I=13694280
Or you can do what I did and draw up your own package:
http://www.xoutpost.com/x5-e53-forum/...installed.html