Here is my experience. I was getting a kind of popping/rumpling sound coming from one of my speakers. It was intermittent, not extremely loud, but with no music playing it was very noticeable. Could not figure it out but ultimately traced it back to one of my mono blocks which was brand new. I started by testing the 6DJ8 driver tubes and sure enough one of the sides of one of the tubes registered a short. I replaced both tubes and the amp is now dead silent.