In reply to:

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It's not the distorsion that kills speakers, it's the associated increase in power - if you go beyond the power handling capability of the speaker.

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What's particularly relevant to mechanical failure is the amount of power to the speaker as a frequency distribution. Speakers can handle a certain amount of power at 1kHz, and a different amount at 2kHz, for example. Distortion causes the power distribution curve to change even though the total power may be constant, and this is what can blow the speaker.

Thermal effects of the supplied current (which is all that matters to the amp) are not frequency-dependent (or not to any significant extent, at audio frequencies).