Originally Posted by SilverEars
Ok, lets say you have two amp of different power output rating. One outputs twice as much power. Also the max voltage much greater with greater headroom for swing.
The one that is less has enough according to specs to give sufficient loudness output for power in. The other have more than sufficient. Would one of these output significantly more distortion than the other? As far as to cause significant affect on the sound output of the headphones?
There's no way to tell without much more detailed measurements. Ideally, you want a distortion vs output level plot at several different load impedance values. It's possible that the higher peak output amplifier has higher distortion at all output levels, or it might have lower distortion than the lower output amp. It might be better at driving low impedance loads, or it might be better at driving high impedance loads. All you can say based on what you said above is that it has a higher peak voltage capability and a higher peak power output (which, if they're from different manufacturers or specified differently, might still not mean much - maybe one amp's peak power is specified at 0.1% THD, and one at 10% THD).
For an example of this, look at the specs on the Denon AVR-X3000. It can put out 105 watts per channel full spectrum (20-20000Hz) at 0.08% THD at 8 ohms, 135 watts per channel at 1kHz and 0.7% THD into 6 ohms, and 215 watts per channel peak (no distortion or impedance figure specified). So does this mean that this is a 105 watt amp? 135? 215? You could argue that the 105 number is the most realistic, since it is full spectrum into a normal speaker load, but the way some headphone amp manufacturers rate their output, this would be a 215 watt/channel amp. At the end of the day, unless the power output of the amp is specified at a specific impedance, distortion level, and frequency, it's impossible to know.
Oh, and as for the final question, any two competently designed amps that can both drive a pair of headphones to adequate volume will sound identical. If a pair of headphones requires 0.5 watts at the peak volume you will ever listen, a good 0.75 watt amp and a good 25 watt amp should both sound the same. If anything, you might have some problems with the 25 watt amp unless you have an adjustable gain structure, since you will be down at the very bottom of the volume range. If it uses an analog volume potentiometer, this can lead to channel imbalance issues. Also, most amplifiers have more THD+N at extremely low output (mostly due to the electrical noise) than they do at moderate output.
Edited by cjl - 5/19/14 at 9:51am