Buy the technic add a sub harness, a cheap mono amp, a good subwoofer and a box from this guy:
https://www.ebay.com/itm/BMW-2-SERIE...72.m2749.l2649
Take it to a good audio shop and have them install it. Takes up hardly any luggage room--especially if you get the 10 inch sub version which I have. And you can take it out if you really need the room by just sliding it out of the side pocket area and unhooking the speaker wires.