I was testing an amp a few years ago to find out the clipping point. I used a test CD with sine waves and an o-scope connected across the speaker cables. I turned up the volume and watched for the sine wave to distort.
I wore shooting-type hearing protectors. Every time I saw the wave form distort, I heard – through the hearing protectors – a hard, grating, irritating sound. I noted the volume control setting.
Then I played music, and every time the volume control was turned up past the “distortion level”, I heard the same hard, grating, irritating sound WHILE WEARING THE HEARING PROTECTORS! This seems to be a repeatable and reliable method to discover the distortion level for an amplifier.
I have used this simple test many times with other preamps, amps, and speakers and then compared the results with the test CD’s tones and o-scope. The correlation is perfect! Check it out.