Other hum/buzz source is the typical 7199 output inverter/drive tube. Some hum, some don't. Now they're very expensive & almost unobtainable, what a PIA to compare.
There's a built in ground loop in many of those 60's Ampegs too, that will result in a low level hum/buzz that won't go away. You'll see a series of black ground wires looping from one eyelet to another, from the power supply and across the circuit board. In self-biased Ampegs I find removing one that links from the ground end of the output tube cathode resistor to the preamp section stops the hum. To check just remove one end of the link wire. Hum gone? remove the wire.
Piazza schematics are (in)famous for inaccuracy.
Welcome aboard MEF and good luck on that Reverberocket from an ex NJ boy, West Orange. Those old Ampegs can sound fantastic when they're working right.