Despite recommendations earlier in this thread to not install the Logic 7 subwoofers with the Alpine sub I did just that.
I can say the upgrade is well worth it, the car sounds really sweet now!
I bought the L7 subwoofers on eBay, they came out of a 335i '08
It was a perfect match, just plug and play.
A small installation tip is to loosen all 4 Torx bolts that hold the chair but DO NOT remove the cables under the chair, next move it all the way to the front.
Thus when you lean the chair back on the rear seat it will be standing quite a bit from the floor leaving you plenty of room to work with...