The app also provides a dosimeter (loudness meter) for supported headphones. Not only does it display the current volume level, but it keeps tabs on your total listening time along with the average volume level over the life of using the app. This can help you determine if you're exceeding recommended volume levels and possibly damaging your hearing. (Note: the app shares this data anonymously with the developer, but you can disable this in the app's settings, if you want.)
I found that CanOpener's EQ options work very well across the board, but the crossfeed effect is extremely variable depending on the type of music you listen to. For hard-panned tracks (common in older stereo music), CanOpener provides a dramatic improvement in the quality of the audio. But the app's effects are much more subtle in music that's recorded with a more dynamic stereo soundstage.
As a music player app, CanOpener's interface is a little quirky, but that's not necessarily a criticism. For example, the progress meter/scrubber is a circle, rather than the straight-line scrubbing control you typically find in other music apps. At first this seemed a little odd, but it actually makes perfect sense if you've used the old click-wheel iPods, as it provides much more control when scrubbing back and forth.
CanOpener is a universal app which means that its interface is optimized for both the iPhone's and iPad's screen sizes. However, on the iPad, it runs only in portrait mode and really doesn't seem significantly different from the iPhone app's interface. It would be nice to see the larger screen used more effectively.
If you have lots of older tracks that need crossfeed tweaks in order to sound acceptable through headphones, or if you have a large library of FLAC-formatted audio that you want to play on your iOS device, then CanOpener is a no-brainer. But if you think your music already sounds fine through headphones, then you may find the app less compelling. However, sometimes you don't realize something needs fixing until someone points out that it's broken. At the relatively low price of $3, there's little harm in giving the app a shot and seeing if it offers some improvement to your music library. Life's to short to listen to bad music.
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