In my case, it was something to do with one of the jacks needing to be cleaned. It may have been the send or return but I don’t remember exactly. It was one of the jacks that intercepts the low level signal when a plug is inserted. When nothing is plugged in (as it always has been in my case) the contacts within the jack are supposed to allow the signal to pass but if it’s dirty, it can cause all kinds of problems. In my case there was some crackle but also a significant loss of output volume. It took just a spray of de-ox into the offending jack to correct the problem.