In fear of looking like I'm trying to prove you wrong - your above statements are not entirely true.
All the factors in having ideal sound reproduction in a car is to generate the imaging & complete sound spectrum of a live recording/ event.
Putting the same speakers in the back of your car as the front will give you too strong a 'rear stage'.
When was the last time you went to see a concert/ band or even orchestra & had the sound coming from behind you?!
A strong front stage is paramount in any audio build - regardless of the environment it is installed in (home, live, car etc).
Rear speakers are called 'rear fill' & they are designed for exactly that.
In just about every install I do, I either dis-connect the rear speakers or leave the stock ones connected - but faded out.
In a perfect world, you want the rear speakers to only be audible by the rear passengers.
You are correct in that bass frequencies (<100Hz) are non-directional.
The sound emitted from the under seat mid bass is NOT subsonic - as it is not a sub woofer.
Mid Bass speakers are designed to reproduce frequency ranges of between approx 100Hz & 250Hz and those frequencies are directional.
Mid range speakers (like our door mounted 4" drivers) typically reproduce frequencies around 300Hz to 3.5KHz and is even more directional.
Tweeters are the
most directional frequency and have a dramatic effect on imaging in a car.
Go have a listen to a passive 3 way system and cover the mid bass with cardboard. You will instantly know what I'm talking about!