OK -- an update. Injecting audio all the way backwards from the speaker connection to the the output of the 2nd detector (pin 4) is clean and works fine. That eliminates the audio chain entirely. Injecting a 455 KHz signal ANYWHERE along the IF chain results in nothing, and the RF gain control, when turned up, is silent. Not a hiss or peep. Voltages at V-1 test good, and exchanging the RF amplifier tube (V-1) for a known good one results in no change. Correct me if I'm wrong, but I believe these tests indicate that there's a problem in the neighborhood of V-1. It seems almost impossible to me that the converter and both IF amplifier circuits would fail in unison, and V-1 and its associated gain control are in common with all those stages. Also, the area around V-1 was the most "diddled with" in the radio. Time for another break until later today, and then V-1 and its associated circuits are going to have to be dissected and compared to the schematic.