Dynamic (voice coil) headphone loads are inductive, rising impedance with frequency. They also exhibit an impedance peak at their resonant frequency. The amplitude and frequency of this will be affected by quality of seal. What we are looking at is likely the effect of back EMF. Headphone drivers are like a microphone too. If we take a headphone and talk into it, we will see voltage at the phono jack. The idea behind back EMF has been discussed much, but we haven't seen evidence, at least I haven't seen evidence of it until now. There were two hypotheses presented here: Current will be the thing that can fight against back EMF. This makes sense from a thought experiment point of view. Voltage is potential. The height of the water tank at the top of the water tower. Current is the actual stuff of water moving through the pipe. We can have all the potential we want, the water tower can be 5 miles high, but it won't do jack if there isn't enough water to push back against back EMF. Global feedback loops muck with the fight against back EMF. I will explore more this later. However I will present the following (tentative, I need to reconfirm results) Note how high-gain has overall slightly worse THD numbers (as would expected because of less feedback), except in the lows where back-EMF could possibly play a larger role.