[RETed]

Wow, since when head units were 2-ohm stable per channel?
Running a pair of 4-ohm speakers in parallel to the head unit that is not 2-ohm stable is either going to blow fuses or kill the head unit.

I would not run the speakers spliced like that anyways even if the head unit could handle, cause you don't have independent control over the (loudness) level of each speaker in the system.
Also, there is this parameter called "damping factor", and it's a valueless number that describes how much control the amplifier has over the speaker itself; when running parallel loads, the damping factor gets split is half - less control means sloppy sound.

As a rule of thumb, the speakers account for the majority of the sound quality of the system.
If you're on the budget, spend the money on the speakers.
Get speakers that are good at reproducing the sound that they are designed for; if you're using crossovers, no need to go really big up front.
BTW, all of the speakers (with the exception of the subs) should be running some kinda high-pass crossover, either passive or active.
Also, spend the money on the front speaker set; rears should be rear fill - no need to spend big bucks for fill speakers.
Also, don't need to runs tweeters in the back or run very large diameters, especially when you already have a subwoofer in the system.
4" to 5.25" is fine; 6.5" is really too big, unless you're just filling the hole.

Next, concentrate on the head unit...
Any of the name brand head units should be okay - even the cheaper models.

Last, (outboard) amplifiers are not that important.
Most of the head units today boost very good power as-is.
Responsibility of the amp should be only to make the signal loud.
I'm not a big fan is building in fancy sound processing - crossovers are a little different though.


-Ted