Pleasurably better, not measurably better

The effect of your mixing is quite similar to the follwoing speakers placements and wiring suggested by one of the members here (sorry I do not recall his name).  The 1st pair of speakers are wired normally and place close together to simulate your center chaneel.  On the 2nd pair of speakers however, the positive terminals are wired normally but the negative (ground) terminals are wired together.  Due to the phase (polarity) opposition, the R2 will only remits the R-L signal and L2 remits the L-R signal.  I tried this setup and, admitedly, I do not find the sound more pleasing than playing 1st pair of stereo speakers alone with proper placement.  The normal setup will render better imaging and soundstage, etc. 

@holmz "To say that the ears are all different would be like saying that the feel of a block of ice or a hot stove is different because all people “feel” differently."

No... we have glasses or custom lasik to correct for differences in vision, some people even have color blindness, and we all have different hearing profiles. If we all had 20/20 vision and hearing, nobody would need correction. As for old and hot, 32 is 32 and 212 is 212, freezing and boiling.... (and even so, some people wear jackets when it's 70 degree weather, while others are loving it in shorts and t-shirt). But that’s not the same as flat to 20hz when my hearing isn’t flat to 20 hz.​​​​​​... If my hearing rolls off at 12hz, then a speaker that rises at that point might actually sound "flatter" to me, at my point of perception, than one that does not rise to compensate for my ears’ rolloff point.

This is why we have bass and treble controls too... to help compensate not only for our own individual hearing but also for our own personal preferences and purposes. It’s music, for crying out loud! Taste matters. I’ve seen plenty of "technically perfect" performances that were boring, boring to me, anyway. There’s something to be said for heart and soul... immeasurable factors. It doesn’t make sense to say one "should" prefer this or that sound, especially if one is reasonably cognizant of audio. Somebody who tells me I should prefer some speaker instead of one that I actually, in usage in my home, like pleasurably better... well, I can confidently dismiss that person’s opinion in that case.

Saying flat is ideal always reminds me of philosophers who have very neat ideal theories which then bump up against real world experience; I’m oversimplifying, but Kant’s "everybody should be treated as if they were all equally rational" comes to mind, and Rawl’s "social justice" theories, as does Marx’s so-called "scientific" economic theories, however compelling on paper... some universal theory of human experience will never, as far as I can tell, be formulated.

Not to dismiss measurements, I check 'em out myself, but technical measurements of equipment don't dictate the pleasure factor of individual listeners... never have, and, as far as I can tell, never will.  The evidence of this claim can easily be seen by the variety of individual choices sophisticated audiophiles make when it comes to our own preferred speakers, for example, in our homes.  In old school terms, "east coast sound vs. west coast sound,"  ... and on and on.  

@curtdr 

It has been suggested previously that reviewers should have their hearing regularly tested and the results should be published for the benefit of their readers but no one seems very willing to do it.

It has been suggested that many loudspeakers have their treble balance tilted upwards in order to catch the listener's attention during a short demo but another consequence of that could be that such speakers will actually sound better to those reviewers who are experiencing some loss of high frequency hearing.

Such speakers will be almost painful to listen to for those who have good high frequency hearing (16kHz+).

Saying flat is ideal always reminds me of philosophers who have very neat ideal theories which then bump up against real world experience;

Maybe, maybe not.

If what we're interested in is the accuracy of playback then we do want a flat frequency response at the point of delivery.

However, there is no good reason for the listener to then modify the signal to compensate for room acoustics, hearing issues, personal tastes etc.

As you say, 

'This is why we have bass and treble controls too... to help compensate not only for our own individual hearing but also for our own personal preferences and purposes. It’s music, for crying out loud!' Taste matters.

It certainly does.

@curtdr If we have someone singing, there is only one singer. Whether one person is there watching or a thousand, it is still only one singer. And it does not matter what every member of the audience is hearing, and what their ears measure at the audiologist.

Saying that “we all hear differently” does not make it become 1000 singers.

We may prefer it pitch corrected, or tone corrected, and we may like the room to not colour the sound.
If I have 10 people listen to the same track, they all say that is Bob Dylan. I do not get people claiming that it is The Pixies, The Police or some random piano track.

Whatever they are hearing, that track sounds like the same Bob Dylan that they have heard and that they know.

@holmz "And it does not matter what every member of the audience is hearing..."

If you’re a member of the audience, it certainly does! It matters to oneself.

Ultimately, it does not matter what every measurement is telling me, if it doesn’t sound great to me, with my ears, in my room, to my taste.

Like someone earlier in this thread here, I am not here to serve the gear, the gear is here to serve me... so matter how "good" or how "bad" the measurable performance, if it doesn’t serve me pleasurably better, then it has no place in my home. Some "audiophiles" really are more "technophiles;" and technophilia has it’s place, but it ain’t gonna dictate my loving audio preferences.

I’ve heard plenty of speakers that "measure better" but that I do not like as much as certain personal trusty pleasurables, so obviously it would be downright silly to buy the less enjoyable speakers just because they measure better! (unless I’m trying to impress somebody other than myself... or if I’m being masochistic or audio-moralistic and insisting to myself that I should like the less enjoyable speakers, and damnit I’m going to make myself like them better because I "should"... )