Another possibility is that a capacitor or diode has failed in the little walwart transformer for the SMS-1, allowing 60Hz ripple to get into the SMS-1.
The "Anta" switching type walwart that came with my SMS-1 says output is 1.25 amps at 12V, tip positive (not sure of the tip size). The back plate of my SMS-1 sys "12V, 500 mA" so a 1 amp walwart should work with good regulation. If you or a friend has a suitable 12V walwart (or you want to risk $20 buying one to try) you could try a swap-out... Take the "Anta" with you to Radio Shack to match the tip size...
EDIT: Never mind. Looking back at what you've tried I missed "hums even if SMS is unplugged."
Do you have a friend with a stereo you could take the SMS over to? If it hums in another system I think the problem is in the SMS...
