From your treble pot description I am assuming that there is no increase in treble as you rotate the control clockwise.
One of the side lugs of the treble pot will be connected to a capacitor... a value of either 270pf or 470pf - Sunn used both and I don't have a 1000s schematic to tell you the exact value. Chances are pretty good it will be a ceramic type of capacitor, rated at least 400 volts or higher, probably light tan in color.
This cap is either bad, or there is a bad solder joint, or the connection has been broken on either side of the cap.