Majority of us don't use rear fill anymore. I have them installed but no amplifier in my Explorer. Just the head unit powering them for the rare time people sit back there and want to hear the music as well.
You can do a active/passive set up. If the mid and tweeter are located close together, you can install those using the passive x-over and use the freed channels for the rear fill. OTOH, try sitting the rear seats sometime - you lose 2-3db of spl. Keep in mind more doesn't equal better and in car audio more often means worse.