I recently purchased one of Andy's Stealth Subs for the 8L A3/S3, now after sorting out my headunit problems I can finally enjoy this setup to its full potential. While i was installing the Stealth Sub I also fitted Focal two ways in the rear and Alpine components in the doors.
The Stealth Sub is a great bit of kit, and certainly performs more like a 10" than the 6" it is. The build quality is excellent and it comes with everything you need to fit it including a great set of instructions. I cant recommend the Stealth Sub enough, I cant believe it took me so long to do this mod, it really is a must have mod for any A3/S3 owner.