Isn't the presumption with most speakers that they will be driven by a voltage source? And by assuming that, isn't it consequently presumed that the amplifier will provide the current according to the impedance while driving the voltage regardless?Yes and yes. But emphasis on **most** speakers; not all speakers expect the amp to be a voltage source. Examples: Coincident Technology, Lowther, Audiokinesis, Pure Audio Project, Spatial Audio, Classic Audio Loudspeakers, pretty much any speaker that is used with an SET (so most horns); high end audio is a diverse community.
Aren't the output stages of amplifiers typically followers? If so, that would make them voltage sources, wouldn't it? If you operated the output devices in a mode that provided both current and voltage gain than I suppose you could call the amp a power source, but that's rare and any reactance in the speaker will exacerbate nonlinearity, wouldn't it?
Not all amps have followers for output sections. Most transformer-coupled tube amps for example do not. Most OTLs however do. Yet both transformer coupled and OTL tube amps can behave as a voltage source if sufficient feedback is applied. IOW its all about the design.