Hi,
I recall the same issue with a Sony AV amp, but cannot recall the model number. Had a similar problem with a Kenwood that I still use. Both were down to dry joints.
If you haven't looked at an issue like this before (apologies if this is an insult...)- use a plastic prod to press around the PCB (s) whilst the unit is on and working. Have a good look with a STRONG magnifying lens; best to remove the whole PCB for this- daylight is best because of the reflections from the solder joints. It takes longer than you would imagine to check all joints on a PCB so patience is the thing. Obviously if you have a scope then a stereo amp repair is a doddle- find where the signal disappears on one channel but not the other. Don't forget to check any speaker protection relays, and the output and input switching. A damp finger introducing hum can be an easy technique, but make sure that the mains only goes to the transformer primary. Some amps have 50V-0-50V PSUs so can bite.
BT