An audio enhancement system can provide spatial enhancement, low frequency enhancement, and/or high frequency enhancement for headphone audio. The spatial enhancement can increase the sense of spaciousness or stereo separation between left and right headphone channels. The low frequency enhancement can enhance bass frequencies that are unreproducible or attenuated in headphone speakers by emphasizing harmonics of the low bass frequencies. The high frequency enhancement can emphasize higher frequencies that may be less reproducible or poorly tuned for headphone speakers. In some implementations, the audio enhancement system provides a user interface that enables a user to control the amount (e.g., gains) of each enhancement applied to headphone input signals. The audio enhancement system may also be designed to provide one or more of these enhancements more effectively when headphones with good coupling to the ear are used.