At the threshold of audibility, the signal and flux density levels in an amplifier with audio transformers are very small. At those levels the relative magnetic permeability of the iron transformer core collapses and the inductance of the transformer becomes very small. The impedances connected to the transformer plus its signal level and frequency dependant inductance behave as a high pass filter which corner frequencies slip into the audio bandwidth, resulting in a non linear signal transfer through the transformer. This research explains deviations in the reproduction of micro details at the threshold of audibility.
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