Every once in a while, when I turn the power switch on on my shuttle 6, I get a very strange crackling noise accompanied with "a fm-radio off tune'' noiselike distortion. Today it happened again and I even noticed a large displacement of the speaker like I never saw it move before while playing through it. I tried different kettle cords, with or without aux in, tuner out ; also with both of my 112 cabs (one has a speakon connector genz benz built in and the other has a speakon cable) . My luthier tried cleaning the jack inputs (all of them, including the fx loop) with a special tool. But the problem still persist. Now I'm reading here that it could be a faulty tube in the preamp section... I have no idea on how to check and replace the tube, I don't know where to look for them inside the amp, I don't know which screws I have to remove to access or whatever, and I'm not sure on which tube to buy. I wrote to Genz Benz a few month ago, but I haven't heard of them. Could anyone guide me in the procedure ? I suppose I could bring the amp to a technician but I would prefer to learn how to do it in case it is needed again in the future.