[MrFixr55]

The hot 7591s give "that warm tube sound". No expert, but I would see what the current is with the bias voltage set per the Tube Spec Sheets, then verify cathode current. Most of these amps have a Cathode Bias C- supply instead of "self bias" (cathode resistor) A popular hack for many amps of this vintage is to put 10 Ohm (correction to original post) 1/4 Watt resistors between cathode and ground for each output tube. These will act as fuses and (hopefully) open instead of the output transformer. BTW, smart move to replace the selenium rectifier and bias circuit resistors. If the C- supply quits, the tubes and output transformer take a major current overload.

I believe that the 7591 was designed by Westinghouse and was one of the last Power Pentodes designed, and was supposed to have better fidelity and less distortion than 6L6, 6V6, 5881, etc.

Very important to note that there is a major difference in pinouts (base diagram) between 7591s and the 6L6, 6V6, 5881, etc.), particularly with cathode / suppressor and grid pin assignments, so it is not a direct swap between these and 6L6s, etc.

OOps!! Edited this to recommend 1/4W resistors instead of 1/2 W Units