I get some major crackle out of my combo amp after it has been outside in humidity. God forbid I should ever play an outside gig or join a band that practices in a warehouse or something. The crackle goes away after about 15 minutes of playing indoors. I guess the moisture burns off or something. Is this an indication that something is wrong with my amp (SWR Workingman's 15)? Does something inside need to be cleaned?