The output impedance of headphone sources plays a crucial role in sound quality and compatibility with headphones, often overlooked by manufacturers. Most headphones function optimally when the output impedance is less than 1/8th of the headphone impedance. Higher output impedance can lead to voltage drop issues, frequency response deviations, and compromised bass performance. The 1/8th rule is a simple guideline to avoid these problems. With the rise of portable music players like the iPod, most headphone manufacturers design their products for sources with low output impedance. To ensure the best sound quality, it’s recommended to use a source with an output impedance of 2 ohms or less.
http://nwavguy.blogspot.com/2011/02/headphone-amp-impedance.html