Grumpy_Mike:
but how does one "decide" to operate at a specific level of distrotion?
You don't it is a function of what signal you put into it. You put in a signal that has a 1% total harmonic distortion then that is the power curve you get. If you put in one with 10% then you get the other curve. If you put in a signal with 20% THD then they are not saying what power you will get.
I don't think that is correct. It is stating two operating conditions that would allow the amplifier itself to ADD
up to that amount of distortion to the signal. Amplifier stages do add distortion, how much is a function of the device linearity and how they are operating on their load lines. Overdrive an amplifier and the amp itself will be the source/creator of the distortion. An amplifiers distortion specification has nothing to do with how much or little distortion is on the input signal. An amps distortion spec is based on only the distortion added by the amplifier.
Lefty