Re: 'Flat' sounding E60
Same problem, expensive car, lousy sound system.
I put in new front door speakers, replacing the stock speakers with Focals and had the tweeters mounted on the front shelf, discretely in the corners.
Big improvement on the sound stage, especially using the radio, but it is still a bit tinny, lacking deep base. Things improve with the volume turned up, but not at lower volumes.
Without spending a fortune, this is about as good as it will get.
BTW, be careful, the supplier fitting the speakers triggered the airbag warning light on my system and I had to get the system checked and reset at BMW - took 4 hours or so and cost $70 (paid by the ice fitter), but could have been a lot worse had the airbag wires been cut.