Axl Fault Line 400b Amp Buzzing?

My sons Bandmate amp is Buzzing when on,I believe even when no instrument is plugged in. As I am 3000 miles away right now all I could suggest was switching head with different speaker body. The buzzing was still there so I basically ruled out the speakers. I will be home soon and would welcome any other suggestions to correct problem. Thanks
Mark Oney, June 2011