That doesn't make as much sense as the distortion/power curves already provided at different frequencise - this tells us how max power varies across the frequency range as shown below. We sometimes find that at low frequencies, less power is available.try to measure frequency response at different power levels.
The same chart gives this information too.We could also try to measure spikes at different frequencies to try to get an idea of the capacity of the amp to deliver the full energy of the original waveform. Does this make sense?
What this chart shows for this amp, is that there is no reduced power at low frequencies. So there is not a weakness here. Supports the theory that the Sonos Amp is set up to boost low frequencies and you are simply used to this, so feel it lacking with a flatter frequency response.