Usually a low hum indicated the 60 hz AC frequency that either isn't getting filtered enough in the power supply, due to a failing filter cap, or a bad ground connection in one of the cables of the unit.
If you have extra cables, try swapping them out to isolate the bad cable, or sometimes you can wiggle the cords to find out which one is the bad one. If all the cables check out ok, you will have to service the unit, and troubleshoot it to locate the source of the noise.
Usually, the bad cord will end up being a speaker wire that has frayed at one end and is making a poor connection on the ground side.
If someone has access to an oscilloscope, you can often see the noise and locate the source, but this usually requires a schematic of the unit to understand exactly where it's coming from so you can isolate the source.
If all the cords and cabes are ok, you will have to get the unit troubleshot. Contact a local repair service, or Fender for the nearest authorized repair facility::
http://www.fender.com/support
Good luck, and hope this helps.