I recently added a subwoofer to my '11 XKR with the Bowers & Wilkins stereo. After a little digging and talking to a few people on here this is definitely the way I would recommend to get a signal for a sub.

Start by removing the plastic passenger side door sill by just pulling straight up on it, you only have to remove the front part as the rear would involve removing more parts that I didn't find necessary. The passenger side kick panel comes off next, also very easy to remove. Just pull straight back and it will come out. Next you will see a thin foam cover on the underside of the glove compartment. It has three push clips right under the glove box door that pull out and then it kinda just falls out. I didnt take any pictures of these steps as they were kinda self explanatory. If you need help with them just ask away and I'll try to help. This pic shows what it should look like so far.





This next step is a pain in the ***. You have to carefully pull the carpet, which is very thick, back to get to the metal plate under it. It sucks and while trying get it back far enough I actually thought about saying to hell with it. There are four bolts that you need to remove to get past the metal plate covering the sub, enclosure, and factory amp. They will be close to their respective corners. Here are pictures of them. The first and third pics I have already removed the bolts.











The bolts on the left are the ones that are the hard ones to get to. Now you will be able to get to the amp for the system and see the factory speaker that attempts low notes ( it is seriously smaller than the door speakers and covered by carpet that would block most sounds ).





There are another four screws holding the amp to the sub enclosure. I removed it to give me some room and see which wires went to the factory sub. I really don't see a reason to remove the amp, you can if you want to. This next pic shows the + and - wires for the sub.




Red being + and black being - like it should be.

This signal is speaker/high level but is controlled by the subwoofer level in the stereo settings, makes it easy to adjust output when listening to different types of music or sources. The amp I am using has high level inputs so I didn't use any type of line out convertor. I also haven't experienced any type of sound roll off at higher levels which at that point I would have added an LC2i since anything else would be kinda over kill for just adding a subwoofer. I did leave the factory sub connected since I didn't see any reason to disconnect it. The system sounds much more to my liking and IMO sounds more complete.

This is just for getting to the signal for adding a sub. If you need help running cables and getting power to your amp there are a countless supply of sites explaining those steps for general purposes.

If I missed anything or anyone has questions feel free to comment.