First of all, @Mad Lust Envy, I'm always happy to see a review from you, man, so thanks for posting!
After reading your review, and then reading your comments above, I decided to run quick measurements of the Sennheiser GAME ONE
...
I thought what follows was a bit interesting. When I first started doing the Sennheiser GAME ONE measurements, I was marveling at how inefficient these headphones are, before I realized they're not inefficient -- I had the headset volume knob turned down. I'd already run measurements with the volume turned down about halfway, so I re-ran them with the volume turned all the way up, and, to my surprise, they were quite different.
Below are frequency response measurements of the Sennheiser GAME ONE with the volume turned down about halfway (dotted line) and turned all the way up (solid line). As above, the first graph is uncompensated (raw), and the second one has diffuse field compensation applied:
Those inline or headset built in volume adjustments use variable resistor in series with driver.@jude, thank you so much for the graphs.
It's interesting how much the lower half of the spectrum changes depending on how high the volume on the headset itself is.
Hence they act exactly like output impedance.
And many Sennheiser have varying frequency dependant impedance with big impedance spike at driver's resonant frequency.
With output impedance that acts as voltage divider affecting to how big part of taken power actually drives headphone.
For example if we take HD598 with nominal ~60 ohm impedance, there's actually 280 ohm spike:
http://graphs.headphone.com/graphCompare.php?graphType=7&graphID[]=2851&scale=30
If output impedance is zero or small practically all power drives headphone.
Now for simplicity if we add 60 ohm serial resistor only half of power is driving headphone.
But at that resonant frequency situation changes to this:
280/(60+280) = ~0.82
And that's 64% increase to how much of drawn power is driving headphone.
As result bass gets boosted.
Also output impedance affects to (electric) damping factor.
Which is measure of how tightly driver is controlled by signal.
Decrease damping factor enough and that boosted bass starts to lose its power and eventually whole sound quality drops down to sewer.
Once tried such inline volume adjusting thing with HD595.