It will level the freqency response of the door woofers like the cap solution. It's added to the front channel like the caps. So the circuit does the same than a single cap. But it it will prevent distortion at higer levels, because it compensates impedance peaks.

Take a closer look at the graphics i've attached. There you'll see how much the volume will be levelled at which hertz. You can also use the tool (link) to calculate your own frequency leveling circuit. So you can decide which low frequency will be dampened and how much it will be dampened.

You could do that also to the 4 channels of the subs. But it would be easier to use a simple coil per sub channel to cut off the subs at -6dB a liitle lower. But don't expect too much. The subs are only 6.5" in diameter and have no defined enclosure (they are free air). The amp only producdes 6 x 22 Watt at 4 ohm. So there are limits by physics.