Tube Sets & single failing tubes

How many folks collecting tube radios have found that a large number of old tube sets tend to fade out signals, have volume issues or similar problems?

You hear a lot about capacitor hum and that's definitely a problem but after twenty tube sets, I'd say that most of my tube sets benefit from a new primary detector/amplifier/AVC type tube. When that's changed out it seems most perform fairly good without much further tinkering.

I think non-tube folks just know the radio doesn't play or tune right and so pass them on for next to nothing even if the repair is a 6 dollar tube.

Am I alone in this "single tube replacement" phenomenon?

On another note, does anyone know of a good site which lists compatible/workable twin diode-triode to triode heptode exchanges?

Thanks for listening.