Another possible causes of the problem is voltage mis-match. I purchased a 220V HA-02 in Australia, and found that it hum and click after running for a while. We talked to Yamamoto about it and they said the amp have to be reset to 240V (which is the Australian local voltage).
I endup returning the amp to the place of purchase. But according to the information send to the local shop, it is a matter of changing the tapping of the power transformer so while you need to open the amp, it should be relatively easily to fix.