Unfortunately i'm not familiar with the guitar, but mains hum is always due to lack of Grounding.
the easiest way to find it is to look at the electrics ( normally under a scratch plate)
With the guitar plugged in and humming - touch a piece of wire from the guitar lead plug body, where it plugs into the guitar(metal) - to all metal components( pots and switches) one at a time and eventually you should find the culprit, when the humming stops.
a lot of modern electrics use metal foil on the underside of the scrachplate as grounding - if this is the case, I would suspect that the high tone switch has worked loose, and is not connecting to the foil.
Good Luck