A nice upgrade for a customer who wasn't at all impressed with the Harmon Kardon factory upgrade this car had. A new source unit is key to good performance in a BMW so an Alpine head unit was discreetly installed with the front panel in the ashtray. Two audiophile Diamond Audio amplifiers power Diamond hex front speakers and a pair of 10" Diamond subwoofers in a custom boot enclosure. An Alpine PXA-H701 processor takes care of all signal processing.