Well, the problem is now sorted!
The replacement driver arrived yesterday, and taking the old driver out reveiled the problem, behold:
i can only assum a small part came unstuck, and then over time speaker movement tore the rest, which would certainly explain why the problem built up slowly. And on comparing the drivers i noticed the newer one was much more extensively glued:
So maby i'm not the first one to have this problem?
and finally, after removing the defective dust cap, i now have a spare 12" neodynium speaker, anyone got an empty 1x12 cab they want to give me?