To my understanding from both hakuzen and amir from ASR's research so far in cables and DACs and/or amps respectively, the lowest possible output impedance possible would be just south of 0.4Ω with cable #125 (57-60mΩ) and the E1DA 9038S (0.3Ω only accurate to tenth of Ω apparently). What use would this have for me I'm not sure. but I assume it'd matter if an IEM (don't remember any headphone go below 24Ω) had single digit Ω of non uniform load impedance like apparently most Campfire Audio IEMs.
output impedance of E1DA 9038S is around 0.05Ω@1kHz, measured with Agilent U1733C. amir doesn't use to measure such low output impedance.
the benefits of keeping total output impedance low are explained in
https://www.head-fi.org/threads/resistance-of-cables-pics-comments-and-links.907998/
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Low resistance is symptom of quality of wires, plugs, and solders (together with total thickness of the conductor, of course).
Low resistance is important to get minimum total output impedance, in order to minimize tonal alterations when using IEM with balanced armatures.
It's also convenient to decrease attenuation and to increase damping factor and efficiency.
1- BA's input impedance curve varies, mainly from upper mids to highs, while dynamic drivers show a flatter curve. Source and cable impedance curve use to be very flat. Due to the BA's impedance curve, the higher total output impedance, the higher tonal alteration. It usually makes the highs harsher, louder, but it depends of the IEM impedance curve (BA's impedance + crossovers), so there are cases where the effect is just the opposite. In general, there is coloration, unpredictable tonal alteration. Total output impedance below 1Ω, and even lower, if possible, is desirable.
2- Also, and this applies to every phones, DD and BA, and speakers, total output impedance higher than 1/8 of the phones resistance (at 1kHz), provokes tonal alteration and higher distortion, specially in lows. This is critical when using very low impedance phones (below 8Ω, for example); in this case (very low impedance phones), you can also reach actual limits of current of your source, provoking high distortion and/or clipping; the source wouldn't like this neither..
3- Also, you get better efficiency with low total output impedance. Louder volume at same voltage. This is good for energy saving, and for longevity of your source. Low output impedance increases electric damping factor, so it helps to drive hard phones.
However, we have to make some considerations about cables.
1- If your source output resistance is 15 times (for instance) higher than the resistance of the cables you are considering, the resistance of the cables becomes insignificant; the culprit of the problem, if any, will reside in your source.
2- The magnitude of the tonal alterations depends of the IEMs mainly. In most cases, the resistance differences between cables won't provoke highly noticeable tonal alteration at human registers; they'll be below 1dB, or even below 0.5dB at audible frequencies, except if we are talking about cables with resistance close or even higher than 1Ω.
It's all about proportions and perfectionism. If we keep our cable resistance lower than 300mΩ (equivalent to 28AWG copper) or 200mΩ (26AWG), most of us shouldn't notice the alterations."
