I also had a similar crackling hissing sound on an SWR WorkingMan's 10.  It ended up not being anything to do with the circuit.  I pulled out the amp module, and found that the little piece of hard cardboard which insulates the speaker wires, and that is popped riveted onto the speaker chassis, had broken off.  In other words the pop rivet had snapped allowing the speaker wires to intermittently short and scape against the speaker chassis. 

Solution:  I carefully drilled out the broken pop rivet and replaced it.  I cleaned and reassembled the amp and speaker into the cabinet and it is now quiet.

A similar thing happened with an Ampeg BA115.  It began buzzing and crackling, and ended up being one of the port tubes had come loose.  Solution: glued it back with silicone, and noise gone.