Originally Posted by vkalia
Actually, while it is true that power is a function of voltage and current (W = V*A), the actual variables that affect power output are the load (impedance curve of the headphone) and the current that the amp can provide at that load (assuming, for a certain volume setting, that the output voltage is constant). It isnt really possible for 2 amps to deliver the same power output and have different current levels, unless the impedance of the headphone is changing from one amp to the other - if 2 amps are providing different current, they are providing different output power.
A powerful amp does make a difference b/c the impedance of even sensitive headphones may not be linear - nor may it be fully resistive in nature. As such, the ability of an amp to handle varying loads does affect the output sound. Which is why a relatively inexpensive, low-current, high-global-feedback op-amp based unit is not going to have the same control over a difficult load (ie, varying impedance, not low sensitivity) as a beefier amp with more current capabilities. In other words, it isnt about the amount of current - it is about ability to handle changes in current requirements due to varying loads - especially once phase issues caused by L/C components of the load enter the picture.
This is as good as it gets and simplest way to explain the relationship with the elements involve that affects power delivery to any headphone.
However, people sometimes just ignore or not aware of this relationships and just throws the idea that more power is better because it's convenient. For an iem especially the multiple-BA types, looking at their sensitivity ratings does not exactly shows the whole picture. The load impedance can vary wildly depending on frequency.
In practice my JH16 has load impedance of 18Ohms against 16Ohms for Roxanne. However, roxanne notioceably requires more power because at the same volume level, the JH16 is louder.
I just wish headphone manufacturers include the minimum and maximum load impedance across the effective frequency response range in their spec sheets.
While i believe a headroom in power available it a must one should be able to draw a line how much headroom is needed for a specific phone. I just feel it's puzzling to see someone recommending speaker amps to power a headphone.