I didn't understand the size of the driver in the subwoofer, but looking at the photo it looks like a 6.5".
Considering a 60Hz crossover, you have four 6.5" drivers that reproduce from 60Hz up and only one that reproduces from 60Hz down.
The radiating surfaces of the two sides in my opinion are unbalanced, they should be at least equal or larger on the subwoofer side.
Of course i'm not talking about "bass boom" but about sound quality and the laws of physics.
If your current situation is fine for you, ok, but you could consider a removable sub box in the trunk with a quick connector and remove it when needed.
Two 8" speakers are enough, and the push-pull configuration allows for small dimensions.
Or a closed box with a flat profile.
Well yes, then you need another amplifier.
Maybe these are steps for the future.