Hum is 60 or 120 Hz noise from the power, and there are two way to pick it up, either conducted or radiated.
Radiated powerline noise is when you have a high current device and it throws out a magnetic field. Any high impedance conductors (your line levels and phono lines) in the field will have an induced 60 or 120 Hz voltage picked up on their wires. You test for this by moving the high impedance wires around. If that causes any change, you have a radiated energy pickup problem. Knowing that, then you can do things like try to improve wire separation (the energy falls off as the inverse of the square of the distance, i.e. it falls off rapidly), or use shorter wires (less pickup area), or re-orient the wires to make them perpendicular to the radiating source (max pickup is when they are parallel to the radiating field).
This radiating magnetic field normally comes from the powerlines or from the power transformer in the high current devices like the final amp power supply.
Trying to shield lines from the 60 or 120 Hz magnetic fields is useless, because at that low frequency the magnetic field cuts through shielding materials very easily. Shielding only works to guard against high frequency pickup. Distance, wire length, and orientation are your best friends.
The other way to pick up hum is by a conducted path. The 60 or 120 Hz noise is traveling through the conductors and getting back onto the input source. If the test above for radiated pickup doesn't work, then you should try electrical isolation methods such as messing with power routing (star or independent outlets) or putting a noise suppressor/filter on the phono power.
Divide and conquer... find out which of the two problems you have first, so you narrow down the type of action you need to take.
