The rear locations for speakers, whilst has plenty of room ( could get 6x9" in ) is real low down behind the seats. I wonder if best to use this space for twin 10" subs and spend a lot of money on just a pair of really good door speakers ?

That's going to be up to you, depending on how much money you want to spend and how much trouble you want to go to for making/finding subwoofer enclosures that will fit the location.

But, in theory, the low-down placement would make full-range speakers perform less than optimally, so your idea for subs+really good door speakers sounds like a good one.

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Tony Fabris