Hi Pete,
The high voltage is after the transformer, not before. At the antenna connector we have around 50 Ohms.
I think it's a combination of current mode and picking the HF up somewhere at the microphone input, I guess. But only Lab599 can answer that and find a solution to remedy this issue.
I use a small CMC at the antenna port, since without, I sometimes got a scratchy voice on 17m and above, due to common mode. I also never used a headset or similar. After about 400 SOTA activations using this rig with different EFHWs, fortunately my audio chip is still alive.
73 Stephan
Stephan HB9EAJ