Actually, I just went thru the process of removing the DSP/Nokia 'subwoofer' from my '00 Beast. That's a whole lot of plastic for relatively output! I crafted a new 'deck' out of 3/4" MDF to house two a/d/s/ 10" subs and an MTX 500-watt D-class monoblock subwoofer amp.
A beach vacation has intervened to stop my project mid-stream, but I will have it all powered up within a week.
For the time being, I am going to rely on the two 4" bandpass ports for entry into the cabin from the trunk. Since mine is a 'free-air' install, I've sealed off all other apertures into the cabin with Dynamat and thinner MDF.
I have yet to take the rear 5-1/4" drivers out of the back deck (that's next), but I will take your instrux with me! I've had two boxes of MBQuart RSB-215s in the basement for over a year to perform the speaker-driver replacement described on
www.bmwtips.com (and eerily similar to Andy Menard's set-up!)
I can't wait to have improved sound--after 42K miles! I'll let you know what I think of the existing 4" ports, and whether they're sufficient.
-Dave