If somebody plugs the output of another guitar amp in the input, it toasts the resistor and perhaps protects other things. If 1/8 watt, much cheaper than a fuse. I got a big discount on my CS800S PA amp because the input resistors were toasted (& nothing else). That scenario is my best guess at what happened. There are lots of 1/4 phone plugs on stage. Some are guitar outputs, some are amp outputs headed for the speakers, which before special Speak-on connectors invariably had 1/4 phone jacks. Garage bands still buy the cheaper 1/4 phone version of amps & speakers.