The car needs to be completely rewired, all the speakers need to be replaced, the sub needs to be replaced, all new speaker wiring, new power wiring for both Bose rear amp and the sub, and the Bose pin on the back of the Concert needs to be earthed change the HU to the flat Bose EQ. The result is pretty lame as well, it's the worst Bose in car system I've ever heard.
For £350 you can have one of my stealth sub upgrades supplied and fitted which takes the stock system far beyond even the Bose upgrade. In fact I've probably sold more stealth subs for A3/S3's with the Bose upgrade already.