When noise is so persistent, sometimes it’s worth investigating cleaning the tube socket and/or pins with some Red Caig Deoxit, making sure to clean up excess after application and fitting a tube in and out of the socket a few times. I would also investigate whether the mic's output jack and cable are fully clean.
You can usually try searching for German sellers on eBay or at “eBay dot de” for EC92, then making sure they ship where you are. Some of the longtime sellers will have tested tubes, though usually not actually tested in mics.
