Since headset failure in CTA can take all the enjoyment out of your flight (and in the USA can lead to litigation) if it's for primary comms best advice is to buy a new one.
Man, in 40 odd years of flying I have never had a headset fail. I have had batteries go flat on ANR models but they still work fine. I have lost an earphone once but the other side was fine. Like when do headsets fail? My experience is they just become a bit dodgy.
I too have some discontinued Lightspeed headsets that became dodgy (but never failed) and it pisses me off that they charge a small fortune for these things then say "sorry; no longer repairable". Bose are no better, I have two of their previous models that are also dodgy but no; sorry "get a new one". Well, I think it sucks and I wish the OP (mikewill) all the best getting someone to fix something that is/was just way too expensive to simply chuck in the bin. Let us know how it goes.
My next ANR will be a David Clark, their reputation is somewhat better than either Lightspeed or Bose.