Having a few extra cents available, here's my two...
While there are impedance standards for headsets and amplifier outputs, they are not always the same, aircraft-to-aircraft. Thus the certification requirement and the different plugs.
One of the biggest causes of "squealing" headsets is use of privately owned headsets bought at a discount from one of the many places that'll sell you such.
If the impedance is too low you'll burn out the amplifier in the interphone system and the first symptom of an amp going south is a squeal. Having too low an impedance is like trying to fill a lake with a thimble. The amp will keep pumping until it has nothing left to pump.
Use the headsets as provided or get an appropriate adapter. Don't aggrivate the avionics techs with writeups they can't duplicate when they show up with the correct equipment after you've been using your modded stuff.