The most common failure is due to the age. This affects the foam suspensions of the Woofer and the Midrange units. After 20/25 years the foam suspensions is simply dissolving; this process  is heavily influenced by external factors i.e: heat, smoke, extrem dry atmosphere… To restore a driver with a  new foam is a MUST to remove the dustcap and to fix the coil within the magnetic gap. Many repairs shops are simply avoiding this procedure  .  The result of such unreliable methods (fast and cheap) is a mechanical stabilty under high power requirements very  far away form the minimum requirements of quality. A replacement of a burned or interrupted Coil is also possible.