Switch the amp off, and turn the knob rapidly from one extreme to the other, several times.
The cause of the crackling is debris on the tracks of the potentiometer. Rapid, repeated, movement usually cleans it off.
If this fails, you can try spraying a small amount of isopropyl alcohol into the potentiometer, and repeating the knob-twirling.
I suppose that you could do it from the front, but you'd need to be careful. Isopropyl alcohol can soften paint, so it might damage the fascia or the lettering.
If you want to try it, stand the amp on it's back, remove the knob, and trickle some isopropyl down the shaft. With a bit of luck, some will find its way inside. If the pot is a dual gang jobbie, getting isopropyl into the rearmost can might be tricky.
I'd remove the cover, and give the pot a good soaking.