iOS: Your iPhone doesn't come with a way to really alter the sound you hear. You can change the volume and balance, but that's about it. CanOpener is an app that allows you to tweak to your heart's content so you can get better audio all around.
CanOpener gives you a lot of options to tweak your audio so you can make your music sound exactly as you wish. You can tweak the crossfeed to get a bigger, natural sound, tweak equaliser settings to adjust your highs and lows, change loudness, and alter lots of other aspects. If that's all a little too much for you, CanOpener also has presets for most popular headphones to get you started.
Overall, there's definitely an improvement to the sound quality, although how much you'll notice (or care) really depends on how willing you are to play with the settings for a few minutes. You do have to use CanOpener as your music app to get the benefit, but it does the job despite lacking some more advanced features.