Today, for my 4Runner's 100,000 mi anniversary I decided to upgrade her sound and bought some Kicker KS 6x9 component speakers for her front doors and a Kicker Key amp to power it all. They also sell a complete 8-speaker kit, so before I bought anything, I listened to my current factory stereo to see how much the rear speakers contributed to the sound. Honestly, it was hard to hear any difference with the rears balanced to the front 100% vs normal 50/50 panning. Maybe 5%? I understand all the rears are running at 8ohms with a crossover and it just kills their volume and sound quality. A quick AI search confirmed my suspicions - and the fact that I don't have party mode - so I only bought the front pair.
For now, I'm holding off on speakers for the rear because the current ones probably work well enough for the quantity of sound they contribute. We'll see how the Key amp DSP changes things once the install is complete. I understand it can actually fix the volume imbalance and restore the crossover frequencies in the rear speakers. If it can do that, I will be very impressed. I'd love to have better sound out of the tailgate.