I have the same problem and I'm not sure if it's fixed.
I have my amp for just 3 months and just last week I noticed the gradual build up of hum and buzz after playing for just a few minutes. It cycles from low hum > loud hum > loud hum and buzz and then popping sound even when all volume is off and no guitar connected. I can't stand it.
I considered sending the amp to a technician but it's heavy and I don't like the fact that I have to pack it up again after throwing the damn box it came with... pff.
My amp never left my room and always plugged in to a UPS (uninterrupted power supply) with built in surge protector.
I tried line6don's suggestion to try diffrent IEC power cable but no change. Tried a different powersource and no change.
BUT TODAY I let it hum, buzz and pop for several minutes turned it off... plugged it back in and the hum and buzz is gone.
Should I send this back? Or is the problem fixed?