I know no one asked, but I’ve got the answer anyway—so here it is:
Yes, a proper 3-way sound system with two subs is possible in an R56, without ruining the interior or sacrificing most of the trunk space.

Let’s be honest: the stock R56 sound system is, in one word—miserable. I went through hell and back trying to improve it. Sure, it can be made slightly better, but the time and effort required just aren’t worth the modest results. So instead, I went all-in and built this monster to replace it.





The Core of the System

Two head units (Raspberry Pi 4 and 5):
– The upper one handles Android Auto and advanced navigation, with a dedicated GPS antenna.
– The lower one is for music, sending a clean digital signal via optical out directly to the DSP.




The system is semi-independent from ignition. It powers on with the car but stays active only for up two hours after the engine shuts off to prevent battery drain.

Independent switches for the amps and screens are mounted on the steering column, allowing me to cut power in seconds. These switches disconnect the REM from the DSP and amps, and also fully cut power to the head units when not needed.
Push-buttons are wired to an Arduino, which sends shutdown commands to the Pis—helping preserve their lifespan.



A small **** under the shifter controls the MAIN volume from the DCP. Since my R56 is manual, and my hand is all the time near the shifter, it works perfectly.

Three buttons in front of the shifter control the DSP and tweeter positioning—we’ll get back to those after the amps.





The Trunk Layout

From left to right:
– 8-channel DSP
– Four amplifiers (two class ABs for the 3-way setup, two class Ds for the subs)
– Power distribution block


The amps are enclosed under a clear plastic cover with two cooling fans mounted on top for ventilation.
The Speakers: The Real Challenge

The biggest issue? Tweeter positioning. For proper sound staging, tweeters need to be aimed at the primary listener.
Mine are mounted on servo motors, so with the press of a button, they rotate toward either the driver or the passenger ( Video: https://t.me/car_audio_hobbit/154 ). Naturally, the DSP also adjusts delays, filters, and EQ to provide the best possible stage for each seat.






Midrange speakers in the doors were a no-go—my left knee completely blocks the factory left-side location. So I mounted them on top of the dash instead.

Woofers are in the standard locations, secured firmly with a bit of 3D printing for proper fitment.

Subs were a bit more adventurous.
The left rear enclosure space is tiny—basically the size of a pipette—thanks to the fuel cap area. I had to search extensively to find a 6.5-inch subwoofer with the smallest VAS on the market. It’s not amazing, but it gets the job done.
The right-side sub has more volume available, so it houses a much more capable 6.5-inch unit.





Soundproofing and Wiring

Needless to say, every panel that could resonate was deadened.
I added extensive soundproofing—starting from the wheel arches and finishing with the roof. Without that, everything I wrote above would be pretty much pointless.

Speaker wiring runs through the top of the door arches, while power wiring stays low to reduce clutter. For the same reason, most of the "smart" control modules are tucked away inside my seat.
If you’re curious and want to see more details, you’re welcome to browse this project diary: t.me/car_audio_hobbit

To wrap it up:
I’m not a sound system pro by any means. I did this purely for the joy of it, as a hobby. It would’ve been way cheaper and insanely faster to pay someone who actually knows what they’re doing. But everyone needs a bit of an adventure, and this one suited me just fine.

 

Nice work, looks like a clean setup.
I'd settle for just having a functional stereo again, without having to shell out $1k for a new HK amp.

 

How did you access/dampen the rear wheel arch panels? i.e. the large, outer surface area between the rear lights and the rear speakers, which is not accessible from within the rear speaker enclosure space. Do you possibly have any pictures of that?

I have found that they vibrate loudly even at moderate volume sub-bass frequencies (or high volume high-bass frequencies) emitted from a full-range 6x9 retrofit speaker (Hertz ECX 690.5, driven by a power amp). Extensive dampening of the inside surfaces accessible from within the rear speaker enclosure space has no effect.

 

I definitely remember that tunnel. First, every piece of plastic interior paneling behind the doors came out of the car, then the speakers and rear lights. The last piece was a removable foam panel inside the tunnel behind the speaker, so I pulled that out too. That gave me probably the best access I could get.
I deadened the metal as much as possible, added a layer of self-adhesive acoustic foam, and put everything back together.
However, there was still some rattling in that area. A few months later, I went back and stuffed the tunnel with acoustic felt. That finally solved it..
I’ll try to find the pictures, I think I took a couple that day, but I’m not completely sure.

Driver’s side was not much better. The space behind the sub was so small that the sub was essentially overdamped (if that’s the real term, probably not). Basically, the speaker itself started rattling at higher volumes because the space behind it was too tight (it was like it tryed to jump out of the magnet). I ended up replacing it with another 6.5-inch sub that had the smallest VAS I could find and that solved the issue.

 

I tryed a plenty of 6x9s. None of the worked, so I 3D-printed a crossovers for 6.5" subs

 

Well, I checked the VAS for your speakers, and I’m guessing that’s the issue. The sub I had trouble with and eventually replaced had a VAS of around 9 liters. The Hertz ECX 690.5 has a VAS of 29. That’s an extremely high number for the limited space behind it.