RST headset-Val tranceiver compatibility

I have not been able to use my RST kit headsets with my Val radio.
Transmission is so low as to be unreadable, reception is not a problem.
I've used the headsets with a Micro Air transceiver, a Collins and a
Icom handheld and the headsets work fine with these radios.
At present I am using a loaned ASA Air Classics HS1 headset with the
Val and am transmitting loud and clear, the Val also transmitted fine using
a loaned David Clark H40 headset.
Has anyone had any experience, good or bad, with this radio-headset
combo? Any ideas?