RPR, e.g. on my Eddie the HU contains a 4-way amp for the doorspeakers, the sub is - like you already wrote - powered by a little amp right beneath the chassis. The 70-5521 allows you to connect the aktive subwoofer to a sub-output on your HU using Cinch-lines.
If your ne HU has enough output power for the doorspeakers, there is no need for additional amps.
In my case, I guess I´ll put an add. hipower-amp (MacAudio ) and Ininity-Spks. in, as soon as I know how to dismount the doorpanels / handles. I don´t mind about the subwoofer after that cause the Infinity Kappa-series are the best I hae ever heared in a 1st. Gen. and 2.nd Gen. Explorer - even without additional sub´s. But they "love" amps with 80 watts / channel or more and my HU only has 4x27 W (52 max.)