I had a pair of Dynaco A-25's that I kept stored in a hot attic for several years. When I decided to re-animate them they seemed fine for about the first half hour. After that they both exhibited scratchy sounds. Pushing gently on the cones with outstretched fingers revealed the voice coils were scraping. Curiously, once they cooled the scraping ceased. Unfortunately it came right back after using them a little. I replaced the woofers with non-OEM ones. It solved the scraping problem but ruined the A-25's tonal balance.
My guess is that during their prolonged non-use some kind of corrosion or rust grew in the voice coil gap. Did your JBL's sit for a good while before you replaced the surrounds?
On the other hand, if your JBL's are making a loud crack like a pistol shot it means the voice coil it smacking against the bottom of its frame basket. Eventually something's got to give. If that's the case you're either playing them too loud or the replacement surrounds may be more elastic than the originals.
My guess is that during their prolonged non-use some kind of corrosion or rust grew in the voice coil gap. Did your JBL's sit for a good while before you replaced the surrounds?
On the other hand, if your JBL's are making a loud crack like a pistol shot it means the voice coil it smacking against the bottom of its frame basket. Eventually something's got to give. If that's the case you're either playing them too loud or the replacement surrounds may be more elastic than the originals.