Cecil Moore wrote:


What is the steady-state impedance of the
source at the fundamental frequency?


Exactly Cecil. That's exactly why I said the concept of impedance is dubious
for large signal devices and thus provides a second argument, albeit weaker,
against the "impedance matching" idea for power amps.

"The concept of 'output impedance' breaks down
for large signal devices. For example, what is
the output impedance of a class C or D amp taken
when the transistor is on or off?"

The strongest argument for dropping the impedance matching concept is PA
efficiency, and therefore maximum signal swing. Obtaining maximum swing is a
load line issue.