Different linearity properties on the positive and negative side would be pretty bad for a speaker, but possible. In the case of a square wave, non-linearity w

Different linearity properties on the positive and negative side would be pretty bad for a speaker, but possible. In the case of a square wave, non-linearity would be identical to a fixed amplitude change though, possibly with a DC bias. Based on the gameboy wiki I looked up, the phase of the 25% duty and 75% duty are such that they are inverse of each other, seemingly eliminating the possibility of combining the two for different waveforms.