Your QuietComfort® 3 headphones have proprietary technology that reproduces a full range of sound, clearly and accurately. There's no need to increase the volume to hear subtle nuances in your music, or to use tone controls, such as a bass-boost feature.
Many portable audio devices have a bass-boost switch to alter audio signals sent to the headphone jack. For the best audio performance, disable bass boost on your portable audio device. Leaving bass boost on may result in exaggerated bass and potential distortion.
If the device is an iPod® or other MP3 player, make sure you set equalisation (EQ) to "off" or "flat" as there's no need for artificial audio enhancements. Refer to the player's instructions for information on adjusting the EQ setting.