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Their is one problem. Take a low impedance source and connect an output impedance increasing adapter in the chain and tell me if you notice the sound gets worse. Because I sure as hell do. The same can be said for taking a source with a modest 10ohms output impedance and connecting a faux output impedance decreasing adapter to into the chain, the sound improves. Mainly in the bass region, I suppose from dampening factor?
And micro details are more evident, atleast to my ears, an in my completely unprofessional test.
I have a good explanation for that. First of all, I don't think what you're experiencing is a matter related to damping factor. It's more of a matter related to voltage distribution between two resistors connected in a single chain, where the two resistors are the headphone itself and the output of the amp. If the output impedance of the headphone amp increases, relatively more voltage is distributed to the headphone amp output while less voltage is distributed to the headphone. Although D7000 is a low impedance, high sensitivity headphone that depends more on current than voltage, the change in voltage can still affect its performance. The reason why you're seeing the improvement mainly in the bass region is probably because that's where the impedance of the D7000 is highest, at around 40Hz (take a look at the impedance curve and you'll see).
Now, I never said output impedance doesn't matter. In fact, I do think having a lower output impedance is always better than having a high output impedance
if every other condition is the same. However, I did not say that an "amp" having lower output impedance is always better than an "amp" having a high output impedance. My point is that as long as the calculated distortion in dB is less than 1, you shouldn't worry whether the inherent output impedance of the amp is lower or higher than one another.
You see, in your experiment, you intentionally altered the output impedance of one same amp. Let's call this amp A. Of course the sound and the damping factor will be altered if you change the output impedance of amp A. However, this does NOT imply that amp B, which has an inherently lower output impedance than amp A, is always the better amp. Why? Because output impedance is not the only thing that is different with A and B. If all amps were designed the same way, the only difference being output impedance, then the obvious choice would be to choose the amp with the minimal output impedance to secure more damping factor and more voltage for the headphone. But in real life, there are countless, far more significant factors that determine the "matching" between an amp and a headphone. Factors that really do matter under the assumption that both amps provide a distortion of less than 1 dB.
Here's a good example: amp A has inherently higher output resistance than amp B. However, amp A also has inherently higher power and voltage output than amp B. In a non-comparison study with amp A alone, the sound will degrade if you intentionally increase its output impedance. Ditto for amp B alone. Which could lead one to believe than an amp having lower impedance is better than having higher impedance. However, when comparing A and B, it turned out that A sounded better. The reason was because A also had a inherently higher power output that made up for the relative loss of voltage distribution that goes to the headphone. See what I mean?
That is why damping factor, which is measured as headphone impedance divided by amp's output impedance, should only be used as a general rule of thumb. It is widely accepted that the damping factor should be greater than 8 (meaning the amp's output impedance should be lower than 1/8 of headphone impedance), but this is to ensure the headphones will get proper damping even when they have very fluctuating impedance curve. For relatively flat headphones like D7000, the required damping factor to ensure a distortion of less than 1dB is far less than the traditional 8, even going below 1 (the fact that WA6SE's 30 ohms output drives 25 ohms D7000 proves this point). What really matters is the equation:
Maximum dB variation = 20 * log [(Zmin+Zamp) * Zmax / (Zmax+Zamp) * Zmin]
(distortion caused by minimal and maximal impedance value throughout the sonic spectrum)