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Could be one of the above causes. But any defective plate resistor in an audio stage can cause the complaint as well. If the volume control has no effect on the volume level of the static then your search can be narrowed to what comes after the volume pot in the circuit path.
When I recap a tube-type anything I also examine all power handling resistors...plate, screen,...whatever, and consider replacing them--especially if they are of the carbon composition type. When they go sour they can produce noise that sounds like an AM radio off station with a bunch of thunderstorms in the vicinity...or noise that sounds like roaring surf at beachside...
Also to be regarded with suspicion are any of the bakelite wafer tube sockets that were in common use in the day. If the tubes involved generated a lot of heat, and darkened the wafers to any degree, the socket became leaky, and such could be determined by wiggling the tubes until a change in the static was heard. Particularly susceptible are output tubes.
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