I have read a few posts regarding the fitting of a sub to a standard concert head unit. I have fitted a phatnoise to my b5 s4 and I am pretty pleased with the overall sound quality but sometimes I feel that it could do with a bit more bass. I am thinking along the lines of an 8" sub in a stealth enclosure. I understand you can take a feed off the rear speakers (I found an article on audiworld). What size amp would I need ?, any suggestions on makes or specific models. I don't want to spend a fortune on this and I would like to keep it as standard looking as possible. Anyone done this before and can share their experience ?