@travlin-easy
Hi Gary,
It's not about certain people in the audience HEAR (or NOT HEAR) the sound "in stereo". Yamaha keyboards voices and effects are stereo, but unfortunately not fully "mono-compatible" as it is called. Many of the voices or effects "break down", so they sound much thinner and/or worse when the two stereo channels of the keyboard are merged to "mono".
I myself have played Yamaha keyboards in
mono at live performances
for decades, even though I have almost always played with stereo PAs. At that time, stereo for me was exclusively the vocal reverb (from an external reverb device or from the external mixer).
In addition to conventional PAs, I've been using BOSE systems for many years now. However, if the available space allows, I also use the latter in STEREO, i.e. I use TWO Bose "L1 Model 2" systems, each with a B2 bass module.
For decades I didn't want to admit it either: Yamaha keyboards just sound "different" (usually WORSE) when they are NOT connected in stereo, but when only the Main Out socket "L/ L+R" is used. The same applies if the connection to an external mixer is made in stereo (two cables), but both stereo channels are not panned in opposite directions on the mixer ("hard left" and "hard right"), i.e. when the panorama (PAN) is set on
both channels to the
middle position.
When mixing
commercially produced music (e.g. CDs or MP3s), great value is usually placed on good, so-called "mono compatibility". Here, with relatively great effort, e.g. special measuring devices such as "Goniometer" plug-ins, the so-called "correllation" is measured, i.e. the phase differences between the two stereo channels are made visible:
>>>
https://en.wikipedia.org/wiki/Goniometer_(audio)
Most of the Voices/Effects on Yamaha keyboards are, as already mentioned, NOT "mono-compatible" by default. And so, in my opinion, it's best to just use the
stereo connection type. It certainly sounds best here.
Of course, in some cases sounds are (intentionally) shifted slightly from the center of the stereo panorama (e.g. the toms or hi-hats within drum voices or complete voices within Styles or MIDI files). To hear this "one hundred percent correct", you actually would have to stay within a certain area between the two speakers, but even if you only hear ONE speaker (and therefore certain drum instruments or individual voices may be a bit quieter because they focus a little more on the other stereo Channel),
the overall sound of a Yamaha arranger keyboard is still better when connected in stereo instead of mono.
One could discuss/write endlessly on this topic. There is no one hundred percent RIGHT or WRONG solution. I can only recommend everyone to
test stereo operation in comparison to mono operation. As I said, BOSE systems in particular still sound very good in many cases in mono operation. Nevertheless, in my experience, Yamaha keyboards also sound BETTER over BOSE if you use two systems and operate them in stereo.

Best regards,
Chris