It sounds like your '06 has the same DSP functionality as the '05...
see this post:
http://www.xoutpost.com/showthread.php?t=3896
The rear door speakers are severely limited in what they play and it seems the system sounds better with the fader all the way to the front.
Since the X5 "DSP" systems lost their digital amp input cable in like '03, and all the digital processing of the CD audio required another conversion from analog to digital (same conversion the radio was always getting), you're one of the only ones who can compare any sound quality difference from before and after. Sounds like the old DSP systems using the cable sounded better? There are a lot of other variables, of course...
I can say that the parts all look pretty good, but they don't seem to perform that way...