Finally splurged on an upgrade to my hi-fi system in my 2009 135i.
Went with a custom built enclosure for the trunk with matching carpeting. 12" JL sub, JL amp, with Hertz speakers up front. I'm a musician so I know music and sound, but the technical stuff of these systems is beyond me... don't really know if I got ripped off or not. Attached is the bill with part numbers/prices, and a pic of the sub enclosure. Let me know your thoughts. Keep in mind there is window tinting in this bill too, and the bulk was the labor to build the fiberglass enclosure. Love the sound up front so far, but the sub may need a little tweaking or something, it's not as tight and punchy as I'd like. Don't really know how all that works...