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This is an interesting question. It goes back to the ( quite faulty ) foundations of stereo as a concept. It can be shown, that the human hearing reacts strongly to inter-aural time differences. That is the difference in time, at which some sound arrives at the left and right ear. The nearly same effect can be achieved by exposing the ears, again left and right to sound of different volume. These differences, time and volume, are perceived as an estimate on the direction, from which the sound comes in. Some physical phenomenon is used by the senses as to construct a picture of the surrounding.
This trained ability of the hearing is used with stereo, but very imperfectly. The often told theory behind it is hilarious nonsense. It assumes a person who is hearing straight forward and never moves its head. The audiophile in this delightfull stereo situation cannot easily be distinguished from a manequin. Actually, some folks tries to follow the rules
O/k, what stereo does is to induce and exploit those mentioned inter-aural differences in the studio as to mimic directional "information" by treating the "signal". But because the theory never holds, the sound engineer / mixer will better test listen to the recording. Only once she is satisfied with her settings, after many correctiions for good, it is published.
Stereo never ever reproduces the original sound field!
Back to Your question. The quite artifical situation of stereo listening is shown to not react to phase lag, excessive group delay, time misalignment, if they are identical in both speakers used. That is a matter of fact. It holds within some uncertain bounds, sure. But the limits are never hit with regular quality designs. Take it for granted, that You will not experience any drawbacks from phase eq. egd eq. tma etc whatever they name it. As long as it is the same for both speakers. It seems the hearing doesn't use the related physical phenomenon for creating a picture of the outside world.
Why is it all so complicated? Because stereo addictives from audiophilia often cite scientific investigations, which are not: not focussed on or even related to stereo. Group delay of so and so is perceptable blah blah and so loudspeaker have to ... (put in some hilariously excessive requirement). Such conclusions are plain wrong. They lack "the middle piece" in their derivations. They secretly assume that stereo has to reproduce the "signal", which is the original sound field. They say, because it is not the original "signal" the "information" is lost. Plain wrong, stereo re-invents the "information" on direction by introducing artificial add-ons to some excerpt of the "signal".
And that can be done and is only done as a difference between the two channels of the program material. As long as the intrinsic phase/volume ondulations of each speaker of a re-producing stereo pair are kept the same, these are irrelevant.
I think you're being a little harsh. Remember, "stereo" doesn't mean "two" or "pair" or anything numerical or quantitative. It's from the Greek stereos, which means "solid" or three-dimensional. The idea was to produce a three-dimensional sonic reproduction of a three-dimensional sonic event. The first experiments were 139 years ago, in 1881. They used two channels, but only because it was a headset-based system. (These trials were public, and attracted what was probably audio's first-ever subjective review, in Scientific American, no less: " ... the sound takes a special character of relief and localization ... to produce this remarkable illusion ... "
Later, Bell Labs experimented with up to 80 channels, before eventually settling on 3, along with most everyone else. Blumlein's insight was to start at the other end of the problem. Delivery was the issue. Two channels was the practical limit. So two channels had to work. And they did. The 90-degree crossed-pair microphones produced hundreds of examples of virtually perfect three-dimensional reproduction. To say, as you do, "Stereo never ever reproduces the original sound field!" is seriously misinformed. Two channels can do it, and I'm pretty sure 80 can, too.