Hi - I have a relatively new SVT-3 pro, but I assume it's no longer under warrantee. Here the question: Sometimes with no bass plugged into the amp, but with the amp plugged into speakers, the speakers make a popping sound every few seconds. You can see the speakers themselves flex or pop out, but the actual pop sound itself is not too loud. I have experienced this phenomenon with multiple cabinets. It does not matter which speaker outputs I use, and I have tried swapping out speaker cables - no effect. It's almost certainly the amp. The problem has occurred in four different locations so I don't think it's a particular building's electrical system. I can usually get it to stop popping by pulling in or out the ground lift knob on the back of the amp. It's totally unpredictable. One day it will work fine, and then the next with the same gear it will not, and then the next it will work fine again. Sometimes it will be fine for a couple of weeks and then it will start again. Sometimes I can't make it stop by changing the ground lift. What is wrong with my amp? I don't want to use it for shows until I get this resolved. Is it just the tubes or is there some kind of grounding/electric issue?