Try a cheater plug on the Hafler
If it is a toroid hum from DC on the line then a Humbuster from PS Audio is a good way to go. But the first step I would take is to open the amp up and tighten the bolts on the power supply's toroid if they seem loose.
But Humbuster and this fix will only work if the sound is coming from the CHASSIS of your amp. If it is coming out of the speakers you have a different issue.
If there is hum coming from the speakers and that hum stays the same volume regardless of where on the dial you have the volume set, you likely have a ground loop hum. A pain, but there are many posts on forums on how to address and diagnose these ground loops.
If the hum is coming from your speakers, try using a 3-prong to 2 adapter on your power cord(float the ground) and see what happens. That may solve your problem.
Some may say it's an unsafe way to run your equipment but I've had several manufacturers tell me that floating the ground often improves the sound by providing a lower noise floor. A couple 'phile friends and I have been doing this for years and almost always hear an improvement.
If the sound is coming from your amp, you may have a bad filtering cap.