DC headsets have had compatibility problems for years, and when I mentioned my problems to an airline friend, he confirmed they also had had problems, so were careful to stick to known pairs which are compatible.
I failed to solve my problems with an HMU, so from memory I returned both headsets to the distributer, who made the necessary adjustments/repairs/replacements of mike? and they have worked fine ever since.
However when another pilot/ examiner tries out his own headset in my aircraft, it inevitably is incompatible, so they have to use one of my matched pair.
Hope this info helps.