Another approach to an upgrade
Really enjoy this forum and the great advice that's being shared. Wanted to pass along my experience with upgrading my '08 M3 sedan equiped with the individual system (yellow cone midranges).
My impressions of the system as stock were fairly positive, except for the lack of deep bass. Imaging, clarity, tonal balance, mid-hi SPL were all fine. My focus turned to bass but didn't want to make anything that's currently good any worse --- specifically dynamics (attack) or tonal balance that can happen when replacing the factory drivers under the seat and adding a trunk-mounted subwoofer.
If I was tempted to replace more, the only path for me would include the new MOBRIDGE piece and new Rockford Fosgate 360.3 DSP with fiber optic input.
Full disclosure: I work at Rockford Corporation and I'm fortunate to work with some of the best electrical engineers at one of the best amp and speaker companies in the world, located a few miles from my home. I also have access to professional audio tools to measure components and systems but tend to use my ears a lot and never, ever, seem to be finished with a system as long as there are knobs to tweak.
I decided to keep the factory under-seat subs but put a Rockford Fosgate T600-2 amplifier in-between them and the factory amp. I had the input of the amp modified to accept 30V ouput from the factory amp. Ran the PUNCH controller to the front of the vehicle and set the HPF to about 65 Hz. This enabled me to adjust the level of mid-bass and really shape the upper end of the low-frequency.
I built a sealed enclosure directly behind the driver's side rear seatback for a P315D2 15" subwoofer, driven by a T500-1bdCP. Set the LPF at 65 Hz and turned subsonic filter on. I ended up using only the passenger-side subwoofer output from the factory amp to drive the T500-1bdCP because I was nervous that the driver's side and passenger's side under seat subwoofer channels had different time delays in the factory amp, causing comb-filtering when summed to a single sub. I never confirmed it, but I'm still running it that way. The T500bdCP was also modified to take the 30V output from the factory amp and the PUNCH controller ran to the front of the vehicle.
I made a bracket to mount both controllers and took some baseline measurements to set polarities, gains and see the interaction of the various controls.
The system sounds great, with great attack and bass definition. The factory part of the system was smoothed a little with the graphic EQ. I will post some measurements next week when I'm back in the office. Here are some pictures of the installation --- the guys at RTTI covered my box and put the very cool logo on.
Hopefully this shows another approach and encourages you to try some different things too. It was a fun project.