And this goes to the very heart of the Mono vs Stereo debate.
TLDR: The core premise and the true value of mono speaker reviews is that the virtues and deficiencies audible in mono is more important than the additional qualities that can only be heard in stereo because those stereophonic virtues alone cannot save a speaker that sounds terrible in mono.
Mono is not "better" per se but rather mono represents the most important stack whereas stereo layers additional qualities to consider. Let's break it down into the virtues of each and why one can co-exist with the other, but more importantly, why mono is a bit better than stereo in predicting listener preferences: (1) Listening in mono offers a pure assessment of what listeners HATE about a speaker - the highs are too sibilant, screeching, "etched", etc. But this assessment does not detract from (2) your sentiment about the unique qualities of stereo listening and how "in real life people don't listen to one speaker" but the "interaction between two speakers... their ability to convey space, etc." So let's discuss these 2 issues separately in order to avoid conflating the benefits and shortcomings of each.
- Mono is the best way to evaluate the flaws and virtues of the frequency response curve and audible distortions of a speaker design. We can all agree that listening in mono allows us to compare and isolate each speaker's individual deficiencies without worrying about stereophonic speaker interaction like soundstaging, imaging, depth, etc. Additionally, mono listening is not limited to on-axis only, but can also be done off-axis at various angles in order to create a fuller assessment of the speaker because speakers are often adjusted for toe-in to improve the sound - old school approach to equalization.
- Mono can fairly determine the subjective appeal of a speaker's FR fingerprint, its signature sound.
- A speaker that ranks highly in mono will have a similar FR curve in stereo. Again, not talking about soundstaging or imaging, just the subjective rating of these qualities that are available in mono: FR, decay, distortion, etc.
- The controversial part: people place more importance on the appeal of these qualities that are audible in mono over the unique qualities only audible in stereo. Let's talk about that next.
- Real life requires the interaction of 2 speakers to convey space, imaging, soundstage, etc. because music sources were mastered for stereo, so therefore 2 speakers are required to assess the "stereo experience". I completely agree that the manner in which two speakers interact offers yet another layer for listener enjoyment not offered by the mono speaker experience (soundstage, imaging sweet spot, depth, etc.). However, the core issue we must discuss is whether these unique stereo qualities are so good that they can overcome the speaker flaws identified in #1 above? Perfect example is the HSU Research CCB8 co-axial speakers which were designed to be listened off-axis (extreme toe-in) because on-axis the highs are unbearably sibilant. The FR curve bears this out:
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However, this was designed intentionally because in order to get the widest phantom channel sweet spot possible, it must be toed in between 15 and 25 degrees which requires that they raise the high frequencies a bit to offset this toe-in so that it sounds flat when listened with this extreme toe-in. But the question is this: would the mono assessment of this speaker have been wrong to conclude that it sounded too sibilant and etched? How many consumers would listen to this speaker without proper toe-in? More importantly: is the value proposition offered by the larger phantom sweet spot greater than that of an alternative pair of speakers with a smaller sweet spot but more pleasant sounding FR at most any position? Given the large selection of $700 speakers, I dare say there are at least 2 others that measure better, sound better in mono, and despite the smaller sweet spot, would be preferred by most people with the exception of the edge case that requires the wide sweet spot.