Gene, W4SZ wrote:
"Why consider the source to be some place after the output conditioning,
such as the output connector, when one can go all the way back to the
wall plug?"

The wall plug can scarcely be responsible for harmonics on the
trabsmission line and antenna, but the output conditionimg can be
inadequate.

A tank circuit of reasonable Q can be adequate to remove enough
harmonics to make the transmitter a linear source in many cases.

A linear source makes King, Mimno, and Wing`s statement on page 44 of
"Transmission lines, Antennas, and Wave Guides" operative:
"When impedances are conjugately-matched for transmission of power in
one direction, they are conjugately-matched for rower transmission in
the reverse direction, if no power loss occurs in the matching devices."

Best regards, Richard Harrison, KB5WZI