I believe at some point I mentioned that the rear upgrade was a waste for me.
The Aussie spec I have is just as dumbed down - no amps and single cone in the doors and 6 inch under the seats (6inch in the 8 inch housing ie real cheap)
I replaced all four doors speakers with the alpine 410's, replaced the under seat speakers with the mentioned earthquakes - blocked the ports an sound matted to death, added a four channel amp in the trunk (in the empty space under the floor ) but then added an alpine PXE-H650 equaliser. I have this set to drive 400Hz above to the doors and below this to the 8 inch earthquakes. Only the front doors and subs are running via the amp/equaliser hence comment that rear upgrades a waste for me. I did not use a five channel amp to also drive the rear doors (2+2+sub) as I needed some higher frequency on the under seat subs hence needed two channel drive - they are really mid subs.
So with just the front door speakers running and the under seat subs it is fully frequency and time delay equalised. ie like wearing headphones in the drivers seat but with thump.
The rears are still running from the stock head unit and are set for very low volume to just provide presence.
All this is completely hidden and looks stock - until you crank the volume :-)
I am sure a dedicated sub in a housing in the rear woud be better but I did not want to lose the storage space and I wanted the car to look completely stock. So I think I have gone about as far as I can go in this respect.
X3 3.0d 2006 (MY07)