You're already using multiple subs, which yields modal smoothing. Adding the car audio sub as a mid-bass-module gave marginally more output (your pic doesn't show the vertical scale) from 50 to 100 hz, but doesn't appear to have helped further with modal smoothing, and there is even some sort of cancellation at approx 130Hz or so, worse than without the car sub in the mix.
So, yeah, it seems like rather fruitless and pointless overcomplexification. (That seems often to be the case for those exploring the use of mbms, typically to compensate for inadequate mains or particularly egregious modal response.)