I've got a 2006 e91 with HiFi and the following equipment: XD600/6, MS8, DLS R4s in front doors, DLS 424 coaxial center (with the other in the pair to posted for sale soon), kicker ssmb8 under seats, and a 12" directed studio sub in the trunk. I've got the xd600/6 in the original amp location, and the MS8 right next to it about 10" away in the sub-floor location. I took out the carpet shell to the subfloor cargo area and Velcro'd the MS8 in the shallow half on the driver side of the car, nearest the factory amp location.
Currently I just have it put together in a working fashion with wires not organized and routed properly, so the floor cover board does not rest flush as the RCA cables make it stick up, but if I can organize it correctly, I plan to mount the XD600/6 next to where the Bluetooth and sat tuners would be as shown in links above, and then put the MS8 in the factory amp location. That way I can regain my storage space. I can provide pictures if it would help, but again, it is not flush nor a final install.