A bit late in the day, March 2017, but....my recently bought Spider Jam off eBay had a really bad hum/buzz with nothing plugged in. Tried different wall power sockets etc and fiddled with the knobs - factory re-set etc etc. No joy.
So - took out the 4 screws on the top plus the 2 on each side so I could get into the innards - fiddled around with the cables - found a ribbon connector which could be unplugged so fiddled with that too. Powered it up again and it's as quiet as a mouse and has been for the last 20 minutes.
If you have a similar problem, don't despair, just do what I did and see what happens (remembering to make sure the power lead is *not* plugged in when fiddling inside!)
PS brilliant amp - really pleased with it.