I would not dream of fiddling with the headset - I know very well the reputation they have got for being well made.
What you say makes sense, I will try and plug it into another plane & mess with squelch/ volume. I have a friend who says he has compatibility issues with the club-planes I fly and his DC headset gets a lot of issues with them. He has a share in another plane he reckons have no issues with his DC so I will try and plug into that next weekend.
Still, please keep the suggestions coming, I really appreciate the feedback.