If the cones have locked solid (they do that in transport if magnets wasn't glued),
the only cure is to replace the woofer with a good used original.
What happens is that the glue that holds the magnet to the woofers metal basket losens
and the magnet will just need a gently bump in either direction to relocate and lock itself
to the outer metal parts, squeezing the voicecoil firmly against the side.
The cone then feels like it has been welded in place.
The same thing happens with other Peerless drivers from that period, Beovox P45 etc.
Martin