So yesterday I did all the tests I can...I kid you not I must've done at the very least 300FR sweeps

All my subs go through a MiniDSP. I have complete control of my subs.
ALL FR sweeps are done with my equipment settings at:
MVL @ -45
LFE trim @ -10
Sub out @ -2
MiniDSP @ -10
(That's ALL negative values)
Amp is a HK 7550HD
Front subs driven by a FP10000Q (bridged and driving 4x 21's at 4200W/channel to an 8ohm load/channel)
Back subs driven by 1 channel of a EP4000 ( NOT bridged driving 4x 18's to an 4ohm load)
SO...My fronts are on 1 channel and my rears are on another channel through a MiniDSP.
My tests had every possible combination: In phase and out of phase, fronts in and out of phase, rears in and out of phase etc...
played with delay from -7.5 to +7.5 in 0.5 increments etc...EVERY single time taking a frequency response!
After seeing all kinds of responses...at the end of the day tried as best I can to match 2 setups:
1. all subs bypassed - playing full range
2. front subs playing 10 to 31HZ and rears playing 33-100HZ.
NOTE...the reason I stayed with the fronts playing the LF (10-30) is because my fronts FR has very good low vs the backs.
The back subs always gave better mid bass (30HZ and up)
Here are the 2 FR responses...I am not saying which is which for a reason...right now...
A the end of this I then proceeded to listen to my goto music and listened to varies songs and all at different levels.
From -20MVL to +15MVL OVER reference.
During this whole time listening to my goto music, I would then switch between both settings (easily done with the MiniDSP)
Finally at 9PM I watched a movie with the wife (Hunter games MJ)
Then a little playtime and finally bedtime
I did not want to post any impressions till the next day (today) after a good night sleep and re-listen to my music with fresh ears!
So back and forth with my settings and listening to my goto music again...
After all that I now have a preference in the 2 settings

It is significant with higher volume!
I hope more people can try this as this has completely opened my eyes on other avenues I will be considering for my next upgrade

I have already named my next upgrade...it's called the "ILMM" (I lost my marbles) upgrade
Kind regards,