front stage picks

I suggest taking the money you were planning on spending on the rear speakers, and instead add it to your front speaker budget.  That way you will be spending the money on the area that will give you the most return.  From there, I suggest you go to the local shops and do some auditioning.  When you find the speakers that sound best to you in your budget, purchase them from that shop. 

For head units, I suggest Eclipse.  I had heard great things about Nakamichi as well, but then found out that the shop I work at had huge issues with their build quality and customer service in the past and dropped them.  At our shop we swap out any defective product with new equipment and send the it back to the manufacturer.  That way the customer isn't without equipment for long periods of time.  But this means that if the manufacturer doesn't cover it under warranty (like Nakamichi was keen on doing) then we would be out the money.  This is much worse if they have a high failure rate.  So we are careful with the brands we carry. 

I have one suggestion for a sub.  Exodus Audio will be releasing a 12" and 15" sub soon that you really should look into.  You won't find a better value on the market.  In fact you would be hard pressed to find a better performer at any price point.  Here's the link: http://www.audiocircle.com/circles/index.php?topic=43520.0

It is difficult to recommend main stage speakers for another person.  What Diamond speakers did you hear and what was it you didn't like about them?

BTW, the new Shiva-X subwoofer is an outstanding driver for the money.

Are you talking about mids for the rear?  Again put as much money in the front speakers as you can as that will get you the most return in performance on your money.  The front speakers are responsible for a majority of the sound you hear and within the bandwidth your ears are most sensitive.  Rear fill IMO detracks from the front soundstage.  But if you have to have it, it should be the last part of your system you focus on. 

An outboard crossover will more than likely be more accurate and flexible than whats built into the head unit or amplifier.  However that will only be handy if you know how to set it up properly. 

You will definitely want to still use the passive crossovers.  You can't just put an active crossover in line and expect the same results. A passive crossover is designed by taking measurements of both drivers (tweeter and woofer) in their intended location and using those measurements to design a crossover that will "voice" the speaker system to the final desired response. There is more than just a set slope at a certain frequency in the design. It also takes into account differing acoustic levels, impedances, and their responses together to create the desired final sound. That will all be lost if you used an active crossover.

I suggest first looking at front door locations for all midbass, midrange and tweeters, as these will all work together to form the stereo image.  Drivers should be as close to each other as you can get them.  The door body, all sheet metal, should be well damped.  And the door panel should be well sealed with caulk and tape.

If the doors are prepped well, you will appreciate the utilization of the ~2 cu ft of door cavity airspace that the woofers are intended to be used in.

Two-way or three-way component sets using the crossovers that they are packaged with is the best way to go.  Two-ways are by far the most common choice and are easiest to set up for good sound.