You can do the calculations if you like. Suppose you have 1V on the amp side of the series resistor going to the headphones. Find the highest impedance in the impedance curve and the lowest impedance in the curve, then you can fill those in as the bottom leg in the voltage divider and calculate the voltage appearing at the headphones, then take 20*log(Vmax/Vmin) to find how much deviation you should have.

The amp you were looking at I think had a voltage divider at the output also, so it may be helpful to start there and work all the way to the headphone driver.