I missed this post, I think it was more widely published over on "the other side" The build looks strangely familiar
I checked out the other site, build doesn't look bad overall.
I do see a classic case of sinking a lot of power and money into subs and very minimal effort (relatively) into the rest of the frequency range, along with equal attention being paid to rear and front speakers. Front stage is always the most important thing in any system, and just plugging in a 4 position system from stereo signals is never a good idea. It creates a higher volume level, but it's just creating noise that doesn't sound particularly "good" to the people in the front seats.
The more I've learned about car audio the more I've gotten with the program that rear speakers are generally a bad idea for cars. There is only one way to do it effectively, which is very complicated, and even that has a lot of debate over if it's the best route.
Those subs will overpower the fronts handily, but a lot of people like that in car audio.
Ben was also saying some strange things about setting amp gains over there. I'm not entirely sure he understands a few concepts, or at least was using a not entirely accurate way to communicate them.
"Nominal" amp gains are just a recommended default setting the manufacturer dumps on there that is really useless. Totally ignore it. Also gains aren't really specifically about the output power. It would be far more accurate to say they are adjusting the input voltage sensitivity. That will relate to the eventual power output, but it's important to understand how it really works when setting up the system. In this case his install is using the audiocontrol LC6, which can create signal voltages much higher than that which any consumer grade amp can handle. You don't need to jack up the signal voltage sensitivity on the amp because the LC6 can bring it up to the high ends of what the amp can accept, and is designed to do it extremely cleanly.
The idea that you just turn up the gains on an amp that doesn't rate up to the power of the speaker is a huge mistake. That's how people under powering speakers blows them. You can't just squeeze more juice out of the amp. It's circuitry is capable of what it was built to do. By trying to get more out of it you'll end up driving the amp into clipping, which sounds awful and kills speakers.
You can use amps that don't rate up to the speakers, but don't use the gains like a volume knob to overcompensate.