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I take this opportunity to ask you what is the best grand piano sound that I can install even by purchasing it to replace the default one which I find too simple...
Most of us have personal (subjective) opinion about "what is the best piano sound". That is, what I consider as good piano sound, maybe doesn't sound right to you. And then there's a question: do you look after "realistic" piano sound or "particular" piano sound.
There's another thing that must be said.. even built-in speakers are
good enough, they can't reproduce the richness of a real piano. Of course we can adjust voice settings to get the best, but by doing that, we actually try to overcome limitations of built-in speakers and so the result maybe won't sound that good on high quality speaker system.
My personal opinion about PSR-SX piano voices.. most of them are just good enough (whatever that means).. or better than nothing. Especially S.Art piano voices, which are tuned to sound "nice" and are kinda overprocessed. I recommend that you try piano voices in Legacy section, because those are less processed and so you can easier adjust them to your liking.
I don't know where to buy hi-quality piano voices which you could install on keyboard. Right now I see two options:
1. You can download free piano "soundfonts" (
here for example), which you can import in YEM and then create install pack which you can try on keyboard. These soundfonts usually unnecessary occupy a lot of keyboard memory and so, if you like some particular soundfont, you can extract samples that you actually need -which makes a voice much smaller.
2. You can buy some piano VST (
this for example) -however that requires PC to be connected to keyboard. Anyway, if you like the sound, then you can also extract samples from this VST and create custom voice (in YEM) -which eliminates the need for PC.
Both above solutions can give you "the best" piano voice, but be prepared for
a lot of work. I recommend you first try to customize one of existing built-in piano voice.
Bogdan