Audio-3D Player Headphones HD 7.1 (free today only) is a long name for an app that can make a big difference in the sound of headphones playing music on your iPhone.
The app is designed to make music sound like you are listening in a room, not through headphones. There are two modes to choose from: one for a 2.1 system (2 speakers plus subwoofer), and another for a 7.1 home cinema system. In the 7.1 mode, you can choose from normal, dynamic, and spectacle. Each succeeding mode enhances separation.
Gallery: Audio-3D Player 7.1