"I removed the earth/ground connection from mains lead on the suspect unit - hey presto - no hum - all works perfectly - except risk of getting fried if goes faulty."



Seems like a ground loop issue if you lift the ground and all is well. If you can't track it down, take a look at some of these products that are made to help these situations-

http://www.musiciansfriend.com/navigation?q=ebtech

And also-

http://www.musiciansfriend.com/navigation?q=power+conditioner

Good luck.

Dan