My 2009 Z4 came with the standard 6-speaker set-up, which isn't that great sounding a stereo. Because it is integrated with the iDrive Navigation system, I decided to keep the head unit and work on improving the speakers. I installed the first upgrade last weekend.
I removed the 6.5" 'supposed' woofers in the kick panels, and replaced them with 6.75" Kicker CompRT Subwoofers (https://www.kicker.com/comp-rt-6-75-1-ohm-subwoofer), wired in series. Although slightly larger, deeper and having a MUCH larger magnet structure, they fit perfectly into the space, though I did have to drill new mounting holes. To power the new subwoofers, I installed a Kicker CX400.1 mono sub amplifier (300 RMS @ 2 ohm) in the trunk where BMW mounts it's DSP amplifier. I ran leads from the wires powering the original BMW 'woofers' into the high level input of the amplifier, and after experimenting, set the gain to about 1/3 (at 2/3 gain, the whole car starts shaking to the rhythm of the music). The bass is now solid and full, such that you can feel the beat of the kick drum.
Next Upgrade - I've ordered EM-Phaser coaxial 4" speakers (150-22k Hz) to replace the midrange speakers in the door panels - they are highly efficient (91dB @ 1M), and should allow music to be played at higher levels without distorting. If this yields the desired sound I'm seeking, I'll stop there. If not, I'll use these to replace the shoulder-height speakers behind the seats and order equally efficient component speakers for the door panels.
I removed the 6.5" 'supposed' woofers in the kick panels, and replaced them with 6.75" Kicker CompRT Subwoofers (https://www.kicker.com/comp-rt-6-75-1-ohm-subwoofer), wired in series. Although slightly larger, deeper and having a MUCH larger magnet structure, they fit perfectly into the space, though I did have to drill new mounting holes. To power the new subwoofers, I installed a Kicker CX400.1 mono sub amplifier (300 RMS @ 2 ohm) in the trunk where BMW mounts it's DSP amplifier. I ran leads from the wires powering the original BMW 'woofers' into the high level input of the amplifier, and after experimenting, set the gain to about 1/3 (at 2/3 gain, the whole car starts shaking to the rhythm of the music). The bass is now solid and full, such that you can feel the beat of the kick drum.
Next Upgrade - I've ordered EM-Phaser coaxial 4" speakers (150-22k Hz) to replace the midrange speakers in the door panels - they are highly efficient (91dB @ 1M), and should allow music to be played at higher levels without distorting. If this yields the desired sound I'm seeking, I'll stop there. If not, I'll use these to replace the shoulder-height speakers behind the seats and order equally efficient component speakers for the door panels.