Well, A&C, I assume you had checked out the design of those intercoms you talk about.
All those I've come across have used the output of the radio as the direct audio drive to the headsets.
That's very different from the intercom amplifier, which should use a separate preamplifier for each microphone position, to overcome impedance matching problems.
I'm not saying some intercoms don't have 4 separate audio amplifiers for the COM and NAV input from the radios, just that I've never come across one that did. But you, as a licenced A&C engineer, should know. Mind you, 757s might be different from PA28s.
Whichever way, if snapper's headset is fine until another headset is plugged in, he has a problem - whether the aircraft has separate amplifiers for each station or, as in this case, obviously not. I've used a wide variety of headsets in our aircraft (which has no COM/NAV amplification in the intercom) and never had a problem.
The microphones, now, that's a different story and is why we bought a new intercom with four separate microphone amplifier channels.