Considering the constraints that KDXSR5 is dealing with, I would probably just run the front speakers directly from the head unit (I don't think an aux amp would greatly improve the overall sound quality). Unless a subwoofer is in the plan you could run the LOW Level RCA inputs to an aux 2 channel amp and simply use it to power the larger rear channel speakers. I believe this would make the installation fairly simple, minimize cost and still yield an improved sound.
The math is correct, circuit type was incorrect though. 
What do you mean by circuit type?
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1989 4Runner: Dual Ultimate (Inchworm front & Marlin 4.70 rear), Marlin Twin Stick, 1200-lb clutch, 4.88 R&P, Aussie Front, Detroit rear, 30-spline Longs, Long hub gears, ARP hub and knuckle studs & 35x12.50 Cooper STT PRO tires. Marlin rear bumper & sliders. FROR front bumper. SAS with Alcan springs & Rancho 9000XL shocks. Budbuilt Bolt-on traction bar. Custom Interior Cage by Those Guys Rod and Customs.
Moab Tested & Rubicon Approved
