I had a custom system put in, ~$6k including a forward-facing Blackvue 650 dashcam, here's a picture (more below):
The box is a custom birch ply sealed enclosure (I think a birch ply enclosure sounds better than standard MDF or HDF). As you can see in the above picture, the sub enclosure is a single unit that also acts as a platform to mount all of the electronics which can be easily uninstalled by removing four bolts and two electrical quick-disconnects (one high and one low voltage). System sounds insane and low end frequency response is ~25hz.
The biggest hurdle when installing and calibrating this system (and any car audio system really) is getting the "tuning" part of the process right. If you have generally good quality components, you can make great sound and don't need a lot of hardware, in fact, less is more in audio I think. Getting crossovers, gain, equalization, and delay right for each set of drivers is key to getting coherence, snap, and musicality in a car environment and making it sound like you're in a concert hall and not a confined space.
Lastly, the tuning process can be supplemented with technology like a DSP, laptop, mic, and RTA application (real-time analysis of the frequency profile of a given acoustic environment) but it won't make the system sound great, you have to use your ears. Most car audio shops will send you home with a system that has too much treble, crappy midrange / midbass impact, and a thumpy localized bottom end. Again, the test should be your ears - know what an acoustic guitar sounds like in person? When you press play in your Tesla and listen to a guitar string being plucked, does it sound just like the real thing? Cymbals should sound like cymbals, not the sound of an in-your-face tweeter in the A-pillar. A kick drum should sound like a kick drum, not thumpy bass coming from the trunk of your car.
Pictures of the whole install:
Electric Fidelity - Tesla Model S | Facebook
Gear list:
Amp: Arc Audio XDIv2 Class D 5-channel amp:
XDi 1100.5
Sub: 12" Arc Audio:
Subwoofers, ARC Series, ARC 12 | Arc Audio
DSP: Mosconi 4to6:
GLADEN DSP 4to6 / DSP 4to6 SP-DIF
Front doors: Morel 6.5in midrange and tweeter:
HYBRID 2-WAY - Morel
Rear doors: Standard Tesla drivers, audio mostly faded forward but a little rear for backfill.
PS: Beware of getting the basics wrong too, e.g. are all drivers functioning and are they all in phase?