https://nwavguy.blogspot.com/201 ... phone-ampsdacs.html
Non-Zero Output Impedance – Output impedance is one of the more common audible differences between anything with a headphone jack. As a simple rule of thumb, if the headphones are less than eight times the output impedance, the amp can alter the sound. So if you have the FiiO E9 amp, which has an output impedance of 10 ohms, you can only use headphones that are at least 80 ohms, otherwise the amp might change the sound. Breaking the “eight times rule” can result in muddy bass and even audible frequency response changes—especially with balanced armature in ear monitors such as Shures, Etymotics, Ultimate Ears, etc. See the Impedance article for more.
