CELESTION SL6 Si - woofer replacement/repair

The woofer on one of my SL6 Si speakers has started "buzzing" on passages where there is heavy bass. Otherwise, I think it sounds fine. I assume the surround needs to be repaired, but that's just a guess. Is this something that can be repaired for not a lot of $? If so, any recommendations? Alternatively, could I just swap in a replacement woofer (if one can be located)? If that would work, any recommendations re: where to find replacement parts? Thanks!