Maybe this is elementary for everyone here but I've seen comments posted that lead me to believe there's some confusion with regards to mono piano patches.
To my knowledge, most pianos are sampled using some variation of the close microphone technique where two mics are placed inside the piano, one above treble strings and the other above the bass. The mics are spaced no more than a meter apart. The close mic technique is preferred because it presents the player, when listening through headphones or properly positioned near-field monitors, a sound distribution similar to the acoustic instrument (treble largely right, bass largely left). Also it captures more of the hammer strike providing more of a direct, percussive sound. More advanced software-based piano libraries often provide multiple microphone positions from which to choose.
Now what are the challenges of collapsing the stereo image into a mono one? Remember that sound travels at approx. 1ms per foot. Let's say, for sake of simplicity, the two mics are equal distance from middle C. In this case the sound will reach the two mics at the same time and there will be no phase issues. On the other hand, what happens if you are sampling a note that resides directly in line with the treble mic? Here it will have almost zero delay in reaching the treble mic but 2 - 3 ms delay in reaching the bass mic. Can this cause phasing issues? You bet.
Below is a recording of a Rhodes note, which was duplicated and moved 2 ms. As you can see there are major phasing issues happening, in fact the fundamental is completely gone if you'd listen to this in mono.
I used a Rhodes above because it's easier to see what's going on with the simpler waveform. An acoustic piano's waveforms are much more complex. Additionally, the two mics don't pick up the same exact sound so the waveforms are different. One might pick up more of the direct hammer sound while the other is picking up cabinet reflections that will probably have more bass content. Together they create a sound that is full, balanced and has depth/image. The phasing issues are ever changing as they are a mix of the note's frequency and the distance sound has to travel to the microphones. There is no simple solution here. If you collapse to mono the stereo image of a piano sampled using traditional spaced-pair stereo techniques you will hear phasing throughout the keyboard.
There are other more mono-friendly mic'ing techniques that could be deployed such coincident pairs and Mid/Side but I don't know of any examples of these in the sampling world, at least as far as hardware instruments.
Why do these digital pianos sound so good through headphones but ‘kinda suck’ when listening through speakers? Below (fig. 2) is a picture of two sine waves that will produce 100% phase cancelation (read: you won't hear anything) when listened to in mono. What's interesting is that if you pan one far left and the other far right, you will now hear the sine waves. If you listen to the panned versions through headphones, the sine waves will sound perfect. If you pan them back to mono the sound will disappear. Headphones allow one ear to be fed 100% of one waveform while the other gets 100% of the other. This is a pristine listening environment where there are zero phase cancellations. Listening through headphones is very close to hearing exactly what the microphones picked during the recording session. In addition, headphones silence nearly all the ambient noise that can interfere with the sound. So the manufacturers don't intentionally try to make their pianos sound great through headphones, it is simply the best listening environment because it is pristine. Unfortunately, it isn't real world.
Continuing with this thought, if you take two great sounding, top-of-the-line near-field monitors and space them properly, you should have an excellent listening experience. But if you place those same speakers side-by-side they will sound boxy, one-dimensional and probably less full. Though I haven't done the experiment myself, I don't think there's much, if any, difference between collapsing a stereo image down to mono in a mixer vs. placing a pair of speakers side-by-side. When both ears are fed the same amount of the two sources, the result will be mono and phasing cancellation will occur. This is the reason why stereo keyboard amps tend to sound boxy and though are an improvement over mono amps given stereo sources, only very slightly.
So what does this phase cancellation sound like when a stereo sample set is collapsed or summed to mono? Below is an MP3 recorded on an RD300GX using the Superior Grand sound. The first example uses the SuperiorMono patch which is a summed to mono example. Two my ears, it sounds identical to the Superior Piano stereo patch when it is summed to mono using a mixer. The summed mono exampled is followed by what I like to call a split mono
. When you think about it, the “stereo” mics used in the sampling process are really just two mono mics spread out. So if you create a split and assign the right half of the stereo wave to the upper split and the left half to the lower, you have a mono sampling of the keyboard but done with two mics not one. This gives a better balance to the sound vs. using one or the other. You of course will need to bring the outputs of the keyboard into separate channels on the mixer and pan these straight up, assuming the keyboard is not able to do this internally. Summed-to-Mono vs. Split Mono
Please note these examples were recorded with no effects and at a static velocity. They are intentionally sterile so that you can hopefully hear the differences.
People always seem to want to sum-to-mono these stereo sample sets and I don’t completely understand the logic. True, one of the reasons why the stereo piano sounds full and rich is because the two mics (often more are used in studio recordings) pick up the tone as it spreads across the instrument. An acoustic grand piano is a massive instrument. If you position yourself at various points around, above and below the case you’ll hear how different the tone can be. It’s those differences, when combined, which create the complex composite tone we associate with the piano. A single microphone, I’m afraid, is just not capable of capturing the same richness of tone. But, as I believe I’ve shown above, multiple mics will invariably create phase issues and there is no easy way around this. It is the proverbial “rock and a hard place.” Personally, I would rather use a mono source (or split mono) that is free from phase issues vs. the summed-to-mono approach.
So do I belong to the Kanker-school? Overall I would say no as I feel the piano needs to be multi-mic’ed to sound proper but I am completely sympathetic to the issues when performing live. Thankfully, my days of worrying about FOH mix, direct boxes and all the rest are gone. When I play out, which has been less and less, it’s smaller, quiet venues so I can set up a couple of small speakers on poles and not worry about much else. When playing live, I do think it's advantageous to use a simpler representation of the piano vs. a more complex/detail version. You need something that efficiently and effectively gets your musical ideas across to the audience. It only needs to sound enough like a piano so that they (and you) are not distracted by it. Given that, I can certainly understand why someone would make the case that a mono piano fits the bill better than stereo. Maybe physical modeling is the answer. I just don't know.
As a side note, I will say the Yamaha AvantGrand, which I own, is remarkable in its ability to mimic an acoustic piano in a room. It has four channels (tweeter + mid-range on all channels) pointing up and four more speakers pointing to the floor underneath. The top speakers are positioned exactly where the microphones were during the recording process. When played at the proper volume it can sound remarkably like an acoustic piano when you stand a few feet away. So I don’t believe one or two speakers are truly sufficient if you’re looking to re-create the image of a grand piano in a room.