Usually a potentiometer goes bad gradually. It gets 'scratchy'.
But if a new unit goes bad (and then another) it suggests there is too much current going through it.
Pots are only rated for 1/8 W, or thereabouts. At 9V, that is only 14 mA.
So when you dial it to either extreme, it can increase the current inside it, enough to cause burning. I have done this more than once. When smoke comes out of the unit, that is how I know when it happens.
So you must make certain whether there is sufficient safety resistance somewhere.
Have you opened up the units, to examine for damage inside?