I have similar views on the system in my 05 X5. It is pretty poor, even with a good source CD.

I pulled off all the door trim last weekend, and was surprised at the lack of a tweeter in the door. Hmm. I have a 6 speaker stereo. I also don't appear to have an amplifier (can't find it anyway). I looked under the CD stacker, and all I can see is the bluetooth module.

Anyway, I'm doing some more research. The door speakers have nice mountings for the stock mid-bass speakers that are about 5". I have some decent focal splits, but am unsure how to retain head unit, integrate aftermarket amp, and wire up splits. The wiring I already have is sending mid+high frequencies to the midranges in the top of the dash, so just connecting the splits crossover to the wiring to the 5" in the door means I'm not getting high frequency stuff.

Anyway, I'll let y'all know what I find. I'm going to make a white noise CD, then hook up a CRO/Spectrum analyser to the feeds in the doors, and see WTF is going on.

The missing tweeters might just be that the stealer took them out to put on another car (like they did to find me a cargo blind), but not sure.

I can't even find a decent wiring+ circuit diagram.

For the record, I have a 2005 e53 3.0d, with TV but no NAV, rear CD stacker, and that's about it. 6 speakers only (only 1 each per rear door too).

I will post any info up as I learn. Any advice is most welcome...

Cheers + thanks,
Scott.