Total output impedance is ra in parallel with Ra. In other words, it's an average that applies for a whole audio cycle. However, at a given point in time the output might be anywhere between Ra and ra. When sourcing current the output impedance is Ra, and when sinking current it is ra. Most of the time we don#'t worry about it, but it can be important when driving capacitive loads, resulting in distortion (slew limiting) if the source impedances aren't equal in both directions.Furthermore (general tube theory question) how is it possible for a gain stage to have a lower output impedance than the value of the plate/anode resistor?