Transformers may be the most linear devices on the planet

Chances are we were conceived while our parents were listening to a song produced with many signal transformers. For example Take on Me - Aha, the lead vocal by Morten Harket sung on a Neumann U47 microphone which has the output transformer right inside it after the VF14 electron tube inside the mic... Anyway the nonlinearity of the metal is exactly compensated as the transformer produces and then reproduces itself. A varying electric field at the primary (the input music) is converted to a varying magnetic field, and then this magnetic field flowing inside the iron is converted back to an electric field at the secondary (the output music) by Faraday's law of induction. Furthermore since by this law the voltage is proportinal to the rate of change of the magnetic field, since increased music frequency means increased rate of change, it follows that as frequency increases magnetic flux must decrease to keep flat frequency responce. So distortion decreases too as frequency increases (only transformers and dynamic (moving coil) or ribbon microphones can do this) so they create very little intermodulations distortion compared to triodes, bipolar transistors or MOSFETs. Further reading: Audio Transformers - Bill Whitlock - PDF