Tube PHONO preamp interference - RFI, EMI, bad grounding?

Ah - that is very telling. Of what, I am not certain, but it may suggest that your input RCAs are not connected (broken solder joint) to ground in their shields, or there is some other failure internally. If the shorting plugs are correct, it should be silent if the only problem here is RF interference. Is the sound coming from both channels? Try connecting only one output at a time (left or right) to see if one side is better than the other.

To make sure you have the shorting plugs correct, check the connection by using your digital multi-meter and touching the DMM probes to the signal pin and the shield on the plugs - make sure you have a positive connection (most DMMs have a connection test feature that emits a sound or beep when there is a connection.). 

I guess there's nothing that could go wrong making the shorting plugs. I simply cut a piece of cheap RCA cable and connected the signal wire to the shielding wire. It still acts as a regular cable though, basically an antenna. 

I measured it with a multimeter - connecting the signal pin and the shield makes a beep. 

If there's something wrong in the EAR itself, how's that possible it's working everywhere else except my home? 😫

There's only one thing that made a difference so far - covering everything - the unit and the cables, mainly the cables - into a metal pot. Or making a thick aluminum sleeve for the input cables. 

Is it possible that there's an interference between the power cord and the output cables? But again. I tried the same cables at different places. Also, the EAR preamp has been powered with a battery. 

I would make sure the EAR is grounded via the power cord.

@filipm 

That it makes more noise with the inputs shorted is telling.

Of course you are living in a high RFI zone; that is why everything works fine elsewhere. Could you clarify this comment:

About the ARC - I have tried the same only with a Bryston B60 integrated, the noise has always been there.

Was the ARC in the system with the Bryston and is that with or without the EAR?

@atmasphere 

I bought the EAR before the ARC LS16 and it's been tested with other things, Bryston B60 was one of them. 

I mentioned it because it's a solid state integrated. When I plugged the EAR into the Bryston the noise was still being picked up. 

There's an interesting point about the ARC though. When I changed the tubes (it's mk1), uses 4 pieces of 6922, the noise was picked up by the ARC too. It was just very quiet and only when the cage was opened. That set of tubes was used, I think it was either JJ or the stock electro harmonix. I'll try if I can make the ARC to pick up the noise again. Anyways, I think it was only apparent for about a minute and then it disappeared. 

I'll have a different set of tubes for the EAR on Sunday. 

@filipm OK. That's 99 44/100% the EAR is at fault. What happens when it is not running off of the battery?