I was thinking about fitting a sub in my A3. Its got the CD changer and Bose option. I was thinking about a 4 channel amp that has hi to low level conversion. So ill take the feed of the rear speakers and run it to the amp as signal feed. But replace the speakers inputs using 2 of the amps channels..and bridge the other 2 for a sub...I know ill probably need a ground loop isolator but has anyone done this. Audi/Bose go on about how every area of the car is tuned for bass from there system and it will mean that some speakers will need the pallarities(sorry spelling) reversed....Is this all true or am i just being silly. Come on then, whos done this allready..