They can measure DP, the pressure drop around the tube anode. Pressure drops around the air paths will be different, and the blowers also will no doubt have different flow rates, for example. Always use the centrifugal blowers. The worst enemy of most electronic devices usually is heat, so these considerations are well worthwhile. The screen supply should be designed with this in mind.
Linear operation requires a steady screen supply.
The screen supply source impedance should be at least 50 ohms in order to protect the screen in case of an arc. A screen protective circuit is designed into this amplifier. The large current emitting surface of the cathode can supply amperes. The tube and associated circuits should be protected by including a series current-limiting resistor of 25 ohms or more. It should be capable of withstanding the high surge-current of an arc or short, and should not be used as a fuse.