I did my KLH-23s that way with no problems back in 2013, and the surrounds are fiber with a soft butyl coating.
KLH has a service bulletin about that.
http://classicspeakerpages.net/library/klh/other/klh_schematicsservice/klh_service_bulletin_60.pdf
KLH has a service bulletin about that.
http://classicspeakerpages.net/library/klh/other/klh_schematicsservice/klh_service_bulletin_60.pdf