@bug,
@The red spirit, I believe you want to look into audio THD (ignore the power formulas, focus on Sine tests) and SPL (sound pressure level).
Total harmonic distortion - Wikipedia
Edit: Prepare to get confused as you read more and more into THD, you want data that includes Sine, SPL and ideally dB measurements.
As far as I understand, there is a amount of amplification that can be done before THD is audible, is different per device.
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I will use my Z906 as an example here, because they give the info in the manual.
Subwoofer: 165 watts RMS (6 ohms, at 52 Hz, at 10% THD)
Satellites: 335 watts RMS (4 ohms at 3.85kHz, at 10% THD)
SPL: > 110 dBC
This should be 99.x audio, 11 THD. However at 100 dBC or lower they may be no audible THD (lower than human audible range, >0 dB).
Also note, audible THD (1 dB or more) can be masked if the pure audio is a higher level, example 99 dB.