Unfortunately, one of my KH120 II units developed tweeter noise after about a year of use. There were reports of the same issue earlier in this thread. It sounds like a quiet static noise even when no inputs are connected, regardless of the switch or knob positions. I tried a different power outlet and repositioning it (thinking it might be an EMI issue), but it doesn’t go away. Strangely enough, it was the same issue shortly right after I purchased it, but it went away for a few months and then appeared again. My other monitor with serial which almost the same is dead silent.
I contacted Neumann, and without asking further questions, they confirmed that it is a hardware defect and asked to contact retailer. Which takes a lot of time, since I am outside the EU where I bought them. Maybe I will ask my local electronics repair service to look at it. There were some skilled guys who repaired my previous Neumanns out of warranty.
I will give up on Neumanns for now. In my previous pair of KH80s with a KH750, one of the monitors broke just after the warranty ended, and the repair technician said that one of the capacitors had gone bad. Just too many issues with each pair