There have been several apps designed to enhance the experience of listening on headphones, using a variety of digital manipulations of the signal. CanOpener is a new entry in the audio app derby, and it's a solid debut.
CanOpener is a US$4.99 universal app with an extensive feature set. A Crossfeed feature lets your headphones sound a bit more like speakers, by blending the separation in adjustable amounts. An equalizer gives you a large range of settings to fit the music to your preferences, and a Dosimeter lets you know how loud the audio is to protect your hearing.
The app also supports gapless playback, and can handle sample rates up to 96 kHz. Channels can be reversed, along with phase, something I don't see in similar apps. It's usually not needed, but it's a thoughtful addition.
How does it sound? Excellent. I listened on a B&W P5 headset, a Sennheiser HD600 and the stock Apple EarPods. The app had presets for the Sennheisers and the Apple product. Listening to rock, symphonic and jazz, I found the presentation improved without setting the aural image to unrealistic extremes. The music never sounded over-processed, which happens with some audio apps. The changes were subtle, but always an improvement over flat response. There is a handy on-screen switch to take all the EQ and crossfeed settings out. One of the best-sounding soundtracks of late is the music from the film Prometheus. It has prodigious bass, delicate highs and great-quality audio throughout. It never sounded better on my iPhone 5.
The GUI is intuitive and attractive, not an afterthought. It's easy to navigate, and that's a plus because there are so many options.
There are two things I'd like to see in this app. Volume is controlled through the iPhone physical buttons, and an odd control on screen. There is a volume control, done with a circular motion, but I don't think it's intuitive or obvious. I also think some additional explanation of the settings would be welcome.
A similar app is MyTunes Pro from SRS labs. It's a free app, but when you add the premium package, it costs the same as CanOpener. The range of audio manipulation is greater in MyTunes Pro, but it is easier to get into trouble and make the sound unnatural. Some people will prefer the MyTunes Pro approach. Others will like the CanOpener approach that is more subtle.
CanOpener is a great debut for a new app in a competitive field. It requires iOS 6 or later, worked fine on the current iOS 7 beta and is optimized for the iPhone 5.