I had to get one of my KH120 II units replaced for a similar issue - after a year of use, the tweeter developed random crackling noises. Although I was lucky with the service and got a replacement from Thomann after 11 days, no questions asked, they simply sent me a new unit with a different S/N.
However, I cannot say that I am absolutely happy with the replacement unit as it also shows a similar pattern of random crackles, although they are very faint and I need to hold my ear up close to hear them. I have tried different outlets, different cables, etc. with no change. So I would not be surprised if this escalates further after some months or so.