Unfortunately everything I tap makes the amp "crackle".     This is why it is so frustrating to isolate the problem! 

Well I did a little more investigating and found that the amp crackles when bumped or nudged.  The strange thing is that I can hear the crackle when ALL the controls are turned down!  Of course it gets louder when the controls are turned up!

This leads me to suspect the output transistors.

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?