In conventional amplifier design, the voltage provided to the output tube elements is only moderately regulated. Of these, the most critical is the screen and control grid voltages. Fixed bias operation helps to stabilize control grid voltage (over that of cathode bias), which the Fisher design uses, so that is a plus. But the screen grids can often draw 5 or 6 times their quiescent current level at full power output. When supplied by a typical dropping resistor then, the screen grid voltage can drop significantly in practice. When it does, the quiescent current setting point drops as well, since the screen grid voltage affects not only instantaneous current flow, but also sets the maximum plate current that can flow through the tube. When the quiescent current is incorrectly set, the "hand-off" of current flow from one tube to the other is not properly aligned, causing distortion to increase significantly. To combat this, manufacturers often (but not always) had to set the quiescent current at a much higher than optimum setting, to counteract the loss in screen grid voltage that typical power supply accommodations produced with increasing power output levels. The thought was that as power output increased, the screen grid voltage would fall, causing the quiescent current level to fall down to an optimum level, all in an effort to minimize distortion at full power output.
The use of EFB™ eliminates all of these problems. In the application of the Fisher 400, the EFB regulators act to hold the relative voltage levels of the plate, screen, and control grid elements tightly together. The main B+ supply will sag somewhat with increasing power output in both channels. But with EFB, the control and screen grids will sag in the exact same relationship with the main B+ sag, that is established under quiescent conditions. Because those relationships hold then under all power levels, the quiescent current can now actually be set at the true low distortion point, rather than one that is much higher that then "falls into place". As a result, the quiescent current level can be set much lower (at the true low distortion setting), allowing for cooler tube operation, and cooler operation of all the power supply components as well. Also, because the optimum hand off now occurs at all power levels, distortion is lowered significantly at all power levels, both low and high.
Finally, when Fisher inexplicably raised the reflected load to the output tubes from 6500Ω to over 10KΩ in later versions of the receiver -- without any corresponding change in operating conditions for the tubes to accommodate the new load -- it placed the output tubes in harms way with potentially serious screen grid overload. So in addition to adding EFB action to the output stage, by setting the screen grids at a more appropriate voltage level for the new loading conditions (creating a new voltage "relationship" for that element), it also resolved the potential screen grid overload condition while gaining all the benefits of EFB action all at the same time.
I hope this helps!
Dave