If, in the voltage amplifiers, there is nearly no current
passing from one stage to the other, in the power stage, the
current becomes important.
The majority of the tubes have an output impedance higher than
the loudspeakers one, a transformer to adapt (=lower) this
impedance is necessary. It allows also to stop the DC which would
be fatal to the loudspeaker, the DC generates only heat and no
noise at all ...
The plate resistance is replaced by the output transformer
primary. The secondary is connected to the loudspeaker.
How DC and AC currents flow in a power stage.