I am really frustrated trying to troubleshoot an SWR Workingman 15 combo bass amp.  The amp makes a crackling sound when jiggled or when notes are played.  I can't seem to isolate the problem.  I have tapped on boards, wires, components and sometimes I can get them to make the sound and other times not.  I even touched up some cold looking solder joints.  What else can I do?