Based on my own personal experience, hum can also be picked up as airborne EMF from a nearby amplifier. In my case, what I had spent hours configuring and re-configuring, as suggested by Tjassoc, was not an AC ground loop at all, but EMF picked up by my IC connecting the phono stage and pre-amp. Simple aluminum foil around the IC remediated the problem. This issue can be very tricky, as my own experience attests to. Good luck.
