Distortion measurement.
Harmonic distortion, measured at 300mV rms input.
The 20Hz figures are not very accurate, maybe plus or minus 10dB.
Frequency....2nd-harmonic....3rd harmonic
20Hz............-114dB............-124dB
1kHz............-116dB............-129dB
5kHz............-107dB............-116dB
7.5kHz..........-106dB
10kHz...........-102dB