Amp troubles! Humming.

Hi there,

I recently bought two Ampeg B2RE's and the problem is this... If I plug my instrument straight into the input they're fine, if I try and go through either my pre amp or my compressor into either the effects return, the power amp in or the main input I get a loud hum. Ive tried different leads, trying the compressor and the pre amp seperately, tried different power leads and sockets. Im all out of ideas to be honest!

Can anyone help at all?