If I recall correctly PR40 cabinets had a "room size" switch, which altered the bass response to suit different sized rooms.?
The bias is not changing since the resistor is between the cathodes and not between the cathodes and ground. Since the tubes are being driven out of phase with each other it will affect the gain of the stage. Kind of a neat hack to change the gain of both sides of the balanced amp at the same time with only a single pole switch..
