Probably the woofer surrounds that have rotted away.
It's fixable by sending the woofers to a refoam'er - or I can maybe help
you to a pair of already refurbished original woofers if needed.
The front grills are glued to the aluminum trim and will lever off by gently inserting f.e. a wide spatula
between the alu trim and the black grill and lift the gril out gently.
The aluminum trim stays on the wooden cabinet.
Work around gently and patiently and it will eventually come off.
Glue it back on using contact glue - just a few drops, not the whole way around or you will
never be able to open them again.
Nice speakers, not too many still around - worth restoring !
Martin