I played a audio frequency cd in 10hz steps from 10 to 200. Her are my observations. I should mention that I have old, very old, ears and hearing to match.

Rear speakers.
I begin to hear at them at 30hz.
Volume peaks at 110 right 120 left.

Door speaker-
Low frequency response is equal in frequency and slightly higher in volume.

Conclusions.
I do not believe that adding filter to door speakers increases the power to or output from the rear woofers but the increase in loudness is due to the reduction in phase cancellation. If phase cancellation is an issue, wouldn't you think the engineers would have addressed it.

Here where I need advice. Since the woofer peaks at 110-120 is this technically the frequency for the filter?
Is there any gain to be had by using a 12db crossover/filter on the door speakers?