I sell WK3000 keyboards at my store and I've always noticed distortion when turning up the volume to high on the piano if you play the keys hard and play more than one note. It seems to me that because of how loud this keyboard is, it doesn't have speakers capable of playing so loud. I don't think this will hurt the speakers over time but it's not an expensive keyboard and Casio couldn't have put a better speaker into this model and charge so little.
George Kaye
Kaye's Music Scene
Reseda, California