Trace Elliot hum

[the celt]

Hi guys,brand new here so please forgive if this is in the wrong place.
i've got a gp7 sm 300 head that hasn't been used in a while,powered it up today and there is a loud hum when connected to the cabs,it's the same with guitar connected or not.
i've searched both site and google and can't find much help.

anyone got an idea what could be wrong.all help greatly appreciated.

 

[Edword]

FWIW, the same thing happened with my GP7SM200. I took it to tech, and he was not able to duplicate the hum. It turned out that the electrical wiring in my house had problems.