I have a set of Polk Monitor series speakers from the late 1980's. These are fantastic speakers in my opinion, even now. However, in one of the speakers (each has 2 drivers) both drivers started crackling very bad. Upon inspection I found the coils dragging on the magnets. The drivers in the other speaker were fine. You can hear the dragging when you press in on the passive bass radiator which pushes the drivers out.
I looked into these drivers with are part number MW 6503 and have been discontinued. There are copies available for $35 to $70 range each.