So; suppose we have a sound reinforcement system, with an ordinary class AB
amplifier, whose manufacturer says it can deliver say 250 watts per channel
or more at some distortion level. And we are using it for a pub band rock
performance, and towards the end of the evening, it's at or close to full
power, with speakers putting out 120dB or so.

Os this amplifier working in Class A some of the time and Class B some of
the time, or what?

Tim