A while ago I made the mistake of cranking my little champ 600 while playing through a EH Big Muff. After playing for a minute, I heard a pop and the amp just started playing screeching feedback. I noticed that the powertube was blown so I replaced that. I turned it on and it is still making the noise, even when volume is at zero! I also had some spare 12ax7's and replaces the pretube and that didn't change anything at all.