I found a Telex
ProCom4 that had independent (adjustable!) squelch thresholds for each of
the four places,


What's really needed is a separate mic amplifier circuit for each station.
Don't know if the Telex has that. Also, some headsets have a little screw to
adjust mic gain. Careful experimentation with that might help, too.

I don't know of any users of PS Engineering intercoms having ever reported
problems of this kind. The intercom built into the Garmin audio panel takes
perfect care of different headsets, too, in my experience.

--
Thomas Borchert (EDDH)