Why are you going to install components in the rear? Rear speakers are only there for fill and, most of the time, should be turned off or down very low. If you leave equal fade between front and rear all it does is puul your soundstage back and down instead of leaving it up and in front in the middle of your dash board. I have a $600 set of CDT components up front running off of a $600 Boston Acoustics reference amp along with a Pioneer 5500 head unit and two 10's in a ported enclosure and, until recently, stock rear speakers. Unfortunately, I forgot I had faded the rear speakers in when my kids were in the truck and didn't fade back to the front after the kids got out. The first time I cranked up the volume I lost one of my stockers. So, I went out and bought a set of Alpine SPS-610's for a whopping $50 and put them in. My point is that I think you are spending money that could be put toward upgrading something else in your system on speakers that should be inaudible most of the time. Some people think their system will be louder if they put more expensive or higher wattage speakers in the rear. In some cases this might be true, but in most cases it is not. Plus with the stockers or a cheap set of coaxials you don't have to mess with running more wiring and finding a place for crossovers and then mounting a tweeter.
All great points, however I really like my sound stage to have a 360 degree presence. As far as the cost goes, I was able to snag those MB Quart components for rear, for a whopping $30 (brand new!!)
1. Crossovers will be mounted on my amp rack
2. The MB Quart system actually comes with a very efficient crossover so I can mount that ANYWHERE... I am planning to place the tweeter from this component nice and high, shooting downwards to fill the cabin. I'm gonna mount is w/out tearing up the fabric or anything. I don't care about that one being flush or not.
3. The JBL amp I bought is more than capable of handling all this stuff that I throw at it.
4. I'm already doing the work to run speaker wire, might as well run it to the rear while I'm doing all of this. At least this way, if I decide against components, I can at least run some alpines like yours or JBL/Infinity coaxs in the rear and hit their true potential by powering them with an amp!
In the end, this is all experimental for everyone right?
We all have different "needs" from our sound systems. We just need to make sure we're truly achieving what we are looking for and not just throwing away resources and getting nothing out of them.
I will say one thing though, After I replaced my HU my stock speakers sounded really good. Then I got the itch to change the speakers. Ended up buying some cheapo Boss for $30 total for front and rear (both 4 way speakers) and I gotta tell you, I did love the detailed sound presentation from the front and rear. What I did miss was the bass. That's when I decided to get an amp and sub. But once I decided to throw an amp in there, I decided what the hell, just get a 5 channel amp (since there wasn't a dramatic price difference). After that, I shopped around and picked up all my gear one by one (painstakingly, trolling websites for the lowest possible prices)