I think kotta's correct.
Many, if not most pots have poor channel balance at very low volumes, but this usually is not present at normal listening volumes. However the crackling is probably dirt, and can be cleaned. The quick way to do this is to turn off the amp and rapidly spin the volume back and forth. The real way involves spraying electrical cleaner into the pot while spinning back and forth.
The real real way is to change to a better quality pot.
Many, if not most pots have poor channel balance at very low volumes, but this usually is not present at normal listening volumes. However the crackling is probably dirt, and can be cleaned. The quick way to do this is to turn off the amp and rapidly spin the volume back and forth. The real way involves spraying electrical cleaner into the pot while spinning back and forth.
The real real way is to change to a better quality pot.