Some parts in a power amp stage get very hot. The output transistors of course but also some of the resistors in the output stage. After years and decades of getting very hot and then cooling and running through this hot, cold cycle repeatedly, some solder joints can develop a hairline crack. Especially if there was a weakness in the joint to begin with. The crack, known as a ‘dry joint’ can give an increased resistance or may be open circuit completely.
Above you can see the track side of a wirewound resistor in the output stage of the power amp of the SX-5580. To fix this, it simply needs resoldering. The second picture shows the joint fixed.