You can just wire the front L and rear L in series and the same the other side and then u running 8ohms no load on amp, equal power to all speakers, don't need another amp, otherwise just run rear coaxials off deck and front stage off amp, the rears are for fill in sound and can still sound awesome.
Currently, in parallel format the effective load (impedance) will favour volume but will not favour cone control and therfore quality will be slightly reduced, to what extent I am not sure, possibly not even noticeable.
In series configuration the impedance will be substantially higher as seen by the amplifier and therefore substantial volume level will be difficult to attain. Which is somewhat counter productive to the addition of an external amplifier.
In both of the above mentioned formats the user loses the ability to fade between front and rear as well as the gains being set equal for both front and rear speakers. This means that you are at the mercy of the sensitivity of the speaker set. So correct staging is almost impossible.
But for the equipment available to them and without tedious wiring, YUBEN and Spiro have done a good job. The addition of another amplifier and some wiring wizardry will make this quite the sound setup.
The part I like most about this subwoofer install, besides the space saving is the isolation of cabin pressure with the mounting mechanism used. With my box setup I simply have my subwoofer aimed at the ski sack opening. This design and placement seems perfect for a free air sub! Would love to have a listen one day and see how it compares to a conventional box.