We just did a basic, stealth audio upgrade for a 2006 X3.
Some pics are here:
http://kward1.homestead.com/2006BMWx3.html
You can see how the door speakers play like 150 and up, and the tweeter plays like 10k and up, and the underseat woofers play like 300 and down - all with dramatic EQ curves.
The OP of the HU, however, is flat as a pancake and goes to 6.8v p-t-p without clipping. (3.4 usable with balanced-compatible amps).
The HU is not as clean as other BMW HUs I've worked with - the last 6-7 clicks on the volume knob are clipping like mad. Keep below that and you're OK.