Don, have you tried Harbeth themselves? Spendor's Terry Miles is now working alongside Alan there and as Terry was the 'wizz' who could properly restore BC1 bass drivers (bear with me), he or Harbeth may now have the skills and maybe time to rebuild old Harbeth drivers. Retrotone can off service old Rogers speakers so again, there just may be a way in to getting HL III drivers fixed.
Tear down these walls; Cut the ties that held me
Crying out at the top of my voice; Tell me now if you can hear me