What you would do to get rid of the hum is to get a transformer with 4 to 8 (maybe up to 16) Ohm on the primary and very low resistance on the secondary.
You then connect your headphones to the secondary side.
For stereo, you need two transformers.
Alternatively: Put a 15 Ohm Resistor (at least 25W rated) in paralel with the headphones.
25W resistor:
Or you buy what @M0N posted.