Amplifier impedance compatibility
The cause of distortion or THD is output current near the limits.

Therefore a higher (32 Ohm) impedance headphone is less likely to be distorted than an 8 Ohm one near max voltage because the current is less. (but obviously less power, of which less is needed for headphones)

However the supply voltage swing available limits the power, so often higher supply voltages tend towards higher impedance output loads to keep the current limitations the same that cause distortion in low power amps.