Are you sure it is coming from the power supply and not an earth / ground loop? Most Wall Warts are rough and ready, unregulated etc and are perfectly fine. Most pedals have smoothing and filtering in them.
Plug the supply in.
Unplug it and as the power supply dies does the hum go ?
I often use a floating mp3 player and my amp hums with it.
If I earth the amp zero volts the hum goes away.
Could be you dont have an earth in the system ?
