The "sound enhancer" is activated when you play music through your headphones or bluetooth device. Two ways to access the settings.
From home screen: settings > sound > sound enhancer. You can choose from several different settings. Tap the down arrow in the upper left, you can choose between urBeats, Solo, or Other. I prefer urBeats. Experiment and find the setting that suits you.
Same settings can be accessed from the stock music player from the now playing screen. Menu > sound enhancer.