So are you saying that if I output a square wave, that would be the worst case?
No there are plenty of waveforms with more harmonics in them than a square wave.
A square wave only contains odd harmonics with a value of the reciprocal of the harmonic number. That is 1/3 of the 3rd harmonic plus 1/5 of the fifth harmonic plus 1/7 of the seventh harmonic and so on up to infinity. So lots of harmonics missing and the power of the higher ones are quite low.
The worst case waveform would be white noise over the full bandwidth of the amplifier, but that is not very helpful as you don't want to listen to that.
In audio there are lots of different measures of power:-