check out Bav Sound (BMW Audio, BMW iPhone, BMW Speakers, Subwoofers, and more | BSW, BAVSOUND, Bavarian Soundwerks.) the offer a full speaker replacement kit. If I recall its about ~800 for an E70 hi fi system. You keep the stock head unit but replace all speakers. Does an amazing job. I did so in my old 545 SMG, as well as my MCoupe. I just bought and X5, and thats at the top of my mod list.