Originally Posted by Nightspore68
Use whichever setting sounds best. I think the impedance of the headphones is actually related to the gain you'll need from the amp. Low impedance headphones need less power to drive. So with low imepdance headphones you could use the lower gain setting. I believe 300 ohms is relatively high, so you might try the medium or high setting. I think. I'm sure someone will correct me if I got it backwards.
I use the medium setting on my headroom amp for my Sennheisers. Now that I'm thinking of it, I need to try the other settings out of curiosity.
Nah, power required doesn't really relate to headphone impedance, but you're right about impedance relating to gain. Headphone amps are more or less like voltage sources, though with some non-negligible output impedance in series for some models, so by P = V^2 / R — power equals voltage squared divided by resistance — if the headphones have higher impedance then they will require higher voltage to receive the same amount of power. i.e. for higher R and equivalent P, you need higher V. Headphone resistance is the real part of impedance, can pretty much just be considered the impedance for many models and at least for simple ballpark estimates. Thus you may need higher gain and volume knob setting to achieve higher voltage, for higher-impedance headphones, to get equivalent power delivered and thus output sound pressure levels. That's assuming equal dB SPL output per mW input (sensitivity), which of course in practice can be very different between different headphones. The gain and volume knobs are pretty much just multiplicative factors on output voltage.
Practically, as mentioned above, just use the lowest gain setting that gets you to your desired volume and adjust from there. You can try the other settings if you want, but usually they shouldn't be much different in terms of performance; mostly higher gains will be noisier, if anything, with maybe a little higher distortion.