Notice that the input signal levels for the highest impedance is twice that of the lowest impedance, and the original curve is intermediate between these. This fixes the max signal. Sometimes the signal at the lowest impedance reaches cutoff. If that happens, choose a maximum level to remove this restriction, and re-derive the points for the other impedances. You can see that the distortion at the lowest impedance is highest, but is comparable at all 3 impedances.
Feedback is one possibility.
I generally have good luck with some slight local feedback from the output stage plate to the driver plate. This does not suffer from a lot of the global feedback effects that shun people away from feedback in general. You are free to go back and start a different set of load lines, to see if you can improve things. In a later installment, we will see how push-pull operation affects the effective plate resistance. In the example we have been using, the quiescent voltage is 300 volts.