The left speaker in my Yamaha electric piano is distorting sound of all notes after about a year of non-use. Right speaker is fine. Headphones fine. Piano is several years old but hardly used. Was stored standing up on its side due to space limitations, could that be the cause? Nothing traumatic occurred like falling over. Any and all ideas about how to remedy this would be hugely appreciated.
Might be foam rot in the speaker. We're just in process of replacing 4 speakers in a Hammond keyboard because of that.
I have a Yamaha Reface electric piano, and the speakers have a mild distortion to them. This seems to be triggered by volume and low frequencies especially. Some of the sounds, such as the Fender Rhodes patches, seem to generate a lot of low frequency heft, which the tiny drivers can't keep up with.
Like you experience, there is no such distortion on the headphone output or line outs.
I think this is just a design limitation; small drivers can't keep up with the signal being fed into them. My Reface YC (the organ emulator) does the same.