Let me demonstrate the distortion issue on example of virtually same headphone amplifier, once with 50 ohm output impedance and second with close to zero output impedance. The headphones used are Sennheiser HD598.

**Headphone amplifier with 50 ohm output impedance**

Frequency response and distortion into Sennheiser HD598

Distortion in % into HD598

**Output impedance close to zero**

Frequency response and distortion into Sennheiser HD598

Distortion in % into Sennheiser HD598

**Conclusion**

Not only frequency response is affected by headphone amplifier output impedance, but also distortion. The distortion depends on headphone impedance non-linearity, on headphone amplifier output impedance and on interaction of headphone non-linear impedance with the headphone amplifier output stage. Measurements performed into resistor or linear RLC load cannot reveal this behavior.