Refurbishing Avalon Ascent Mk 2

After 25 years of service my Avalon Ascent Mk 2 have started to distort especially with classical piano.  I have isolated the problem to both 2" aluminium-manganese mid drivers.  These, I believe, were built by MB Quart of Germany, now out of business and the part seems unavailable.

I would greatly appreciate any suggestions on how I could bring back these speakers to their former glory. Thanks.