I noticed that Solo Synth and Hex Layer can sound very louder than Drawbar and PCM tones. PCM and Drawbar tones only have the master volume, while Hex Layer and Solo Synth go beyond: they also have individual, layer volumes, because they allow you to have multiple layers.
Let's suppose we play the wave called "Electric Crunch Guitar 1", in PCM mode, with master volume at 127. Now, we play the same wave in Hex Layer mode, with master volume at 127 and layer volume at 0 (mid point). You won't hear a considerable loudness difference. Now, we repeat the test, but change the layer volume to +127. Then, that difference appears. It's like amplifying the tone; multiple-layer tones allow you to "amplify" the waves. I don't like this, but you can solve the problem: by assigning the layer volumes to 0 when creating your patches. If the tone comes from the factory, copy it and apply such change. Hex Layer and Solo Synth tones made by CASIO can sound lower or louder than the other types, because the layer volumes.
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XW-P1 and this topic are about 10 year-old, but such keyboard is still in use and and still being sold -- although the stocks might be very small now. I bought mine some months ago. I didn't find the answer in the forums and the handbook, or at least I didn't understand it. Then, I had to think and try different things. The text above is the conclusion of my experiment. I hope it helps other people.