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When in doubt follow the recommendations of the Amp manufacturer.
The last 2 I bought advise using a DMM on AC and a 0 DB sine wave at XX frequency depending on what your powering.
Since the target voltages in their chart are based on a 0 DB sine wave and not actual music I recommend using the recommended source with the recommended target.
Sine wave is important, not just frequency, because it affects load voltage.
Also if your doing gains by ear it's much easier to detect the clipping distortion threshold with a sine wave form as the texture of the sound contrasts much more audibly with the full waveform than it would with a non-sine or more square waveform, which is much closure in texture to what clipping sounds like to begin with.
Granted if you had a scope you could push the envelope, but if you got the right amp for the job, you shouldn't need to push it. You should be conservative and still have alot of extra power unused. At least that's how I approach it. I'm not competing, I'm drinking a beer at the beach or commuting, I want loud and clean, but mostly clean.
Disclaimer: I'm not an expert, I am here to learn.