Regarding setting the impedance selector it should be noted that iFi Pro iESL has, efectively, as far as the gain is concerned, only two significantly different setings; those for 96 and 68 ohm - and those for 24 and 16 ohm. The difference between the first and second group in gain is just below 6 dB - which is significant ( almost double as loud ) . DO NOT switch from 96/64 to 24/16 ohms while music is playing loud - in case of a more powerful amplifier, damage to headphones may result. Reduce the volume setting by at least 6 dB prior switching impedance from 96/68 to 24/16 ohm setting. Switching in reverse direction will only result in lower output level, no danger to headphones even if you play loudly without reducing the level. The differences in output level/gain between the 96 and 64 ohm is barely audible 0.2 dB or so - about the same for the difference between 24 and 16 ohm. The changes in level are not significant, something that can not be said about the resulting frequency response, where the differences are audible. Furthermore, the level of distortion in the low frequency range, which happens due to magnetic saturation, is significantly better with high imedance settings . Ultimately, it is the combination of amplifier and your ears/preferences in music/sound which will decide which setting will be most enjoyable by the end user. More about this in my forthcoming , long ovedue review of this extraordinarily interesting "transformer". I have to decide which of the ( too numerous) measurements should be presented - in comparison to Stax SRM1MK2 amplifier, both Pro iESL and Stax driving a single pair of Stax Lambda Pro. Stay tuned !