These are now getting very old, and the bass drivers are at the top. Because of their age now the bass cone suspension could very well be starting to sag and cause the voice coil scratch in the gap (it’s called polling).
They have 4 screws undo them and turn each bass driver 180 degrees and remount them, hopefully this will cure your problem.
http://img.photobucket.com/albums/v223/mhardy6647/timewindow.jpg
Cheers George