really stubborn mains hum

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Hi goodguys,

The test we suggest you to do is to short-circuit the inputs and see if there is still noise. It corresponds to the test craftsmen do in your house when they hear a noise they cannot figure out where comes from.
They close all doors and windows to a room to hear if the noise is still there when the noise cannot come through the doors or windows. A trivial and logical test actually.
Shorting the amplifier inputs corresponds to closing the door to a room such that no noise can come into the amplifier (or room) and it will show if the noise is generated by the amplifier itself (or by something inside the room).

When you tell us that you can use an amplifier with shorted inputs (that is like passing a door when it is closed) and suggest that amplifiers should be delivered with inputs shorted (corresponding to delivering houses with the doors bolted), our engineering logic is put to a hard test. Either there is an important misunderstanding or we simply talk different and incompatible languages.

I believe it was Scott who guessed on a ground-loop. Ground-loops can be very difficult to find the origin of, even for experienced engineers. They cannot just be refuted on a gut-feeling unless you are very experienced with such ground-loops.

As said by others and if you still wish us to help looking for the cause, please follow the advises you are given. Many of us have several decades of experience with electronic problems.

Shorted RCA-plugs may come from use at unused amplifier inputs such that if you accidentally switch the input selector to an unused input, you avoid the noise that the unused input may pick up. Shorted RCA-plugs are NOT used on active inputs as they prevent any source signal from entering the amplifier.
 
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