I'm having a crackling issue during our last few practices so I decided to take my head home last night. Thinking back my best guess is it only happens when I first turn on the amp and goes away after some long period of time 10-20 minutes. I only hear it when making noise on my bass and don't think it's sensitive to any playing dynamics or frequencies. I have a laundry list of test to try short of buying new tubes like new speakon cable, clearing out dust, using another head for the pre/poweramp to isolate the circuit, etc etc.... Apart from the obvious tests - what other diagnostics can I try? I should be able to get access to any electrical equipment I need. Could anyone direct me to more in depth resources?