Has anyone fitted an external amp for a sub on the Audi MMI stereo? How to get the best sound quality? Via a high to low or is it worth spending money on the advanced high to low's if so recommendations please? Where is best place to get feed from on the MMI? Front or rear speakers? or is there an easier place than that? Also is anyone aware of the wiring diagram for the speakers i.e. colour codes to + - L and + - R ?