Back to the magnetic fields, remember that a guitar pickup is *supposed* to convert magnetic fields to electrical signals. Grounding, shielding, aren't going to fix that, nor should they. Humbucker pickups (think Les Paul) are less sensitive to these external fields, but are not completely insensitive. Single-coil pickups (Strat, Telecaster, etc.) are particularly vulnerable. Transformers, motors, some lighting, some wiring can generate these fields. If the hum changes when you change the guitar orientation then this is not a grounding problem -- you are picking up these fields. Perhaps there are additional sources of noise, but you do have to address the magnetic field problem.