Those inline or headset built in volume adjustments use variable resistor in series with driver. 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.