I'm not an audio expert, but I have been actively involved in the design and build of every home and car audio system I have ever used. I'm an engineer that works in the manufacturing and production of new flight hardware so I certainly understand all the theory. That being said, I've always thought that most of the electronic gear out there is WAY overpriced. I visited an online parts mega-store looking for inexpensive components that work well together. Let me explain myself a little bit.
Let's take BOSE as an example. My opinion about this company is that they make decent gear. It is overpriced in my opinion, however Bose engineers to a really good job with tuning the gear they use. In general they use cheap Chinese manufactured components that perform very well within a limited range. Outside of that range they are crap, where most other speakers would still be performing well. How Bose fixes this problem is by using electronic cross-overs to simply restrict the poor sounding frequencies from ever reaching that particular speaker. Then they just add another cheap speaker into the system to cover the frequency ranges the first speaker is not capable of reproducing. This is why most Bose packages come integrated with a bunch of speakers and a head unit that contains all the amp and cross-over circuitry. They essentially "hard wire" the tune of the system into the electronics and then sell the package. This kind of approach has it's place - for people that just want out of the box sound they rarely ever have to tweak.
So all of that being said, I am taking a similar approach. I'm purchasing very inexpensive gear and then tuning it to sound as good as it can within the ranges it is best at reproducing.
Let's start with what I have. My 1995 Miata came from the PO with an older aftermarket head unit, aftermarket door speakers, and the rear channel and trigger wires already wired into the trunk. When I first bought the car, the stereo didn't work well at all. No headrest speakers (fine with me) and both front speakers were crackling in and out - just couldn't even listen to it at all. One of them has a broken speaker surround. The first thing I did was cut all the connections, re-strip them and solder them, along with some glue on the broken surround. Viola - the front speakers are working just fine now. Next up was installing a very cheap amplified sub in the trunk since the PO was nice enough to already have wires run for me. It has a built in crossover and volume control so I just played around with it until I stopped getting any kind of bass distortion. As of today, the system works just fine. I listen to CDs every time I'm in the car.
After doing a little thinking about my approach, I visited Parts Express. For about $225, I purchased all of the following:
PAC adapter
electronic crossover
4 channel x 100W amplifier
2 x 5/14 door speakers
2 x 4x6 rear speakers
speaker wiring
Power/ground wires
fuse holder & fuse
2 x power/Gnd distribution block
And here is how I planned it out. Just to see how it sounds, I plan on keeping the front speakers powered by the head unit. I'll send the rear channel signal to the crossover where it will be split between the rear speakers and the sub. Before going to the speakers, it gets amplified. The sub is already amplified.
And the power/ground/trigger plan
After I get it all wired up I'll see if the front speakers need to be routed to the amplifier first. I have a feeling I'll be able to get decent front/rear balance by playing around with the fader controls. If there is too much of a disparity between the front/rear, then I can change the wiring to power the front channel off the same amp the rear channel is using.
The only advice I could use is on my battery. I have an OEM battery and I'm close to needing a replacement. With the addition of the extra electrical loads, I was thinking I should get the battery at the same time as the install. Was thinking about an Optima but open for suggestions? Is an OEM battery in good condition good enough?