Cause no one had shielding problems with these monitors before, mine are very stable. It's a very curious case you have here - you have problems with these speakers nobody had before and also should not be.
Speaker produce relatively high harmonics at low frequencies and it's possile to hear that with a sine signal. But your other speakers should have the same effect.
Software sine generators are not always clean enough! Check your source signal, RME digicheck could help? Or put the wave signal in your sequencer and do FFT analysis.
Maybe buy a pair of fitting, new cables when you rebuy the speakers. Something like the Sonarworks ref mic should be fine for your measurements and not to expensive. Or you just use the MA-1 which is anyways a recommended addition.
Well, I had that kind of interference problems. And was weird...but both monitor had that shielding problem (maybe one a little bit more than the other). I noticed that because I put my smartphone on the desk. And the monitor started to buzz and to do weird noises, in a very noticeable way. With my old monitors this happen if i put a device really near (like 1 cm), but not at 30/40 cm like the neumann. Even the pair of cheap mackie I use for my tv don't get interferences.
So you say it is normal for this speakers to produce high harmonics at low frequency (like playing a sine at 60 hz)?
I know that the signal is clean because I always used a frequency analyzer in the channel.
I used a fiew brands of cable, one was a brand new cordial with neutrik gold connectors.