The guy i buy my speakers from (INO audio in Sweden)( it was he who made the Swedish test of the 14B SST) is working with a speaker with back firing sound.
Looks like this:


The back speaker is a 10" Woofer and a Tweeter.
The front Woofer is delayed the same amount as the distance between the front and back Woofer so they play in sync towards the listening position.
The back Woofer plays up to 300Hz and is non adjustable.
the tweeter comes in softly from 4000Hz and gets stronger all the way up to 20000Hz and is adjustable all the way down to off position.
All depending of the room and distance to the back wall.
The upward firing back speakers are very important.
He does not want any midrange in the back, he has worked on this project for decades and this is his solution.
I am not an expert but try to explain as good as i can from the thread in a Swedish forum.
Here is the complete thread for those who understand Swedish

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https://www.faktiskt.io/phpBB3/viewtopic.php?f=20&t=66195