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

Is there any test I can do inside the apartment? I invited an electrician and he said the power line should be fine. 

Beautiful city Prague.

If you are in an older building, you should get an electrician to verify the male earth pins in the building are in fact grounded. They may not be, or the grounding may be compromised with poor connectivity - corrosion etc.

Also if both the EAR and ARC are grounded through the power cables, then when you connect the 2 units with an RCA cable then you have an earth loop - have you tried lifting the ground on the EAR ( making sure iit is grounded via the ARC by putting a multimeter across the 2 chassis.?

OP,

Congratulations on the ARC REF150. Great sounding amp.
 

Now, I would recommend you give up on your EAR and get a ARC PH8… or better yet a ARC REF Phonostage. Each additional piece of ARC is incredibly synergistic… culminating in a really amazing sound with all ARC. Although it makes sense in retrospect, I was really surprised when I dropped in the final ARC piece in my system.

@dover I already got an electrician over and it was the first thing he did. I demonstrated the problem with the phono preamp, he even brought a 12V battery and we had plugged the EAR into that. I have shared a picture in a one of the previous posts. 
Maybe I'll have to call him again, or I'll find someone else to be sure. I will consider getting the tester to do it myself. 
The building isn't that old, it's in the center but it's a functionalist building from the 50's. The power line could be 20-25 years old. I'll open up one of the sockets to take a look. The voltage is pretty stable too. The owner takes very good care of it.
I'll definitely call the electrician and make sure he checked it properly. But I think we even checked the grounding outside of the apartment. 
There's a different circuit in the kitchen, I tried to power up the EAR from there, no change nor difference in the noise level. I could run a power cable from a socket outside of the apartment.
I have finally bought proper power cables, they are on it's way. 
As soon as I sell the Classé power amp I'll get better interconnects too.
 

I have mentioned this issue in the conversation with the ARC staff as we were discussing the advantages of the ref150 over a ref110 (the use of KT150 tubes) and they basically said I should get the best interconnects I can afford and keep them short. 
 

And yes, I have tried lifting the ground in the EAR's power cord. 

@ghdprentice  the original idea was to find an ARC PH5, still affordable but all the ARC products are rare in Europe. So is BAT gear or Conrad Johnson. I have read how EAR compares to PH5 and I simply went this way.  
I'm a bit afraid I'd have the same issue with any ARC phono too. 
The REF150 is great, it gives a lot of room for improvements in the rest of the system. If I get an opportunity to get a good deal on an ARC phono I'll go for it.  
 

@filipm if you are going to open up a wall socket to check things out, might as well check for DC voltage on your AC power too - just put your DMM to DC volts and the probes on hot and neutral - I don't think it would cause this issue as it typically causes mechanical transformer hum and vibration and not noise in the signal path. However, I suppose the tubes could be picking up that vibration if present via microphonics - and it is another thing that might not be present in another location.

If tips from phono stage manufs including Jud @ Joule-Electra(RIP), Kevin@K&K and everyone here couldn't resolve it, it was time to move on. 

@sbank If that was a JE phono section, the input gain stage was so starved for current that it easily rectifies RF energy and so will be prone to RFI problems. I think Jud didn't want to increase the current to a more linear region as gain was going to be sacrificed.