The 8” are not full enclosure housings which is why they quit at higher levels. The JBL woofers are free air so you would have to rebuild the box for to install an 8” sub on each one and then you still have to cover your lower midrange unless you want the subs to handle it.I hear the sound system is a disappointment. This seems very tricky. Looks like all small speakers. From the looks of it, i think door speakers are 5 1/2. And the two in the dash are for like the infotainment.
I think i would do a three way active up front. Coaxial in the rear for music fill in. And then maybe a 8 inch from jl. The stock sub enclosures look too small to create good bass with a different sub.
I’m getting mine back Monday (hopefully) with a rebuilt box. One 10” JL sub in the center and two 6” woofers for fill for what the 4”s can’t handle. You will need to take apart a good deal of the interior for this: half of center console, b pillar interior from the roof to floor along with the seats.