People rarely make 'maximum power' measurements with speakers, partly because of possible damage to the speaker, partly because the speaker's properties may change as it heats up, but mostly because the sound level is unbearable for our ears. Some people say "But you
must measure with a speaker because that is how the amp will be used". Others say "It will never be used continuously at a single frequency and maximum power".
All speakers have different impedances. Some testers have used dummy loads which have impedances more like speaker impedances. Here is one example, albeit quite a complex one
https://sound-au.com/project216.htm.
Cheers,
GJ