@thor The volume of the hum is the same regardless of the volume nob? .. hmm thats unlikely to be RFI then

could be the power supply for the system, or a ground loop. If it is coming from outside the speaker it is traveling throught he power line then not the audio lines. RFI wouldnt be powerful enough usually to do much throught here which is what makes me think its a ground-loop issue or internal to the speakers.

Would require some more info to really narrow it down.. anyway if you need help let me know.