A: It is possible that a vacuum tube in the unit is going bad. While preamp tubes tend to last much longer than power tubes, they can still go bad, be defective, or be damaged through misuse or mishandling.
Another possibility is that the gain pot is dirty or going bad — we would suspect this if you’re hearing the crackling while the pot is being turned. Before you start pulling out tubes and replacing them, try cleaning the pot — or even just work the pot back and forth vigorously for a bit — to see if this reduces or eliminates the problem.
It is also possible that some other component is dirty or failing. If cleaning the pot and swapping out tubes does not solve the problem, a trip to a service center may be required.