I just finished installing a BSW X108 Subwoofer / Amp combination in my '04 X5 3.0 (non-DSP).

The installation was a bit more involved than I anticipated, particularly as it pertained to locating the Rockford Fosgate amplifier. I found many threads on this forum to be very helpful with late-night questions. I'll try to post photographs later, and perhaps my experience can be helpful to others as well.

The improvement in sound quality is truly monumental. I'm thoroughly impressed - this upgrade is worth every penny IMO.