The app is tweaked for certain headphones, including the Beats by Dr. Dre, the Bose OE2, iPhone 5 Earplugs and the Koss Porta Pro 2, plus the Sennheiser HD 238, HD 650, Momentum, the PX 200 II and the PX 90. There are more brands and models matched to the app, and you can check them out on the developer website.
I tried the app on my Sennheiser 650s, and music sounded really good, especially in the 7.1 mode. I couldn't hear sounds from directly behind me, but the rear surrounds were certainly behind the front and side surrounds. For classical music, rock and jazz, I thought the Audio-3D player made the sound more dramatic and "live" sounding.
The 2.1-channel mode sounded much less full, but the app has adjustments that let you set the distance of the instruments, and you can change the apparent direction of the sounds. The app also features a bass boost option.
I tried the app on some phones not on the recommended list, and they sounded excellent as well. For example, my B&W P5 headphones came alive and I preferred listening through the Audio-3D player. The developer is taking suggestions to integrate headphones not on the approved list.
When you first start up, the app will ask for access to your music, and you'll get complete control, including album covers if you have them in your iTunes library. There is also a terrific-sounding test track, and you can select individual virtual speakers to hear directionality.
Audio-3D Player sounds good and for now the price is right. Apps that perform a similar function include the Dirac HD player, and from SRS Labs, My Tunes Pro.
Audio-3D Player 7.1 requires iOS 5.0 or later and is optimized for the iPhone 5.