This is caused by a small amount of DC voltage in the mains which makes its way through to the mains transformer, it will become ‘saturated’ and start to hum like a trooper.

The DC Blocker is a clever device that you place at the IEC (at the rear power inlet) and as its name implies, it stops DC from getting into the amplifier. It therefore eliminates amplifier transformer hum.
Many large amplifiers have a really annoying mechanical hum (emitted from the toroidal transformer itself).
You can hear it when you place your ear close or next to the chassis.