The equalizer (EQ) in iTunes 11 is a stellar tool for tweaking frequency ranges in order to optimize sound for your particular set up. However, there have been EQ issues in the past that have caused users to experience problems like sound distortion. With the latest version (11.1.3) of iTunes, Apple has announced a fix along with a set of other improvements.
According to Apple's knowledge base on version 11.1.3, "This version of iTunes resolves an issue where the equalizer may not work as expected and improves performance when switching views in large iTunes libraries. This update also includes additional minor bug fixes."
It's interesting to note that Apple have also fixed the speed issue when switching view modes in iTunes 11 when dealing with a large music library. If you're experiencing both problems, then upgrading could be the answer.
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- Improving Sound Quality in iTunes 11 Using the Equalizer Tool
- How to Use iTunes Sound Check to Normalize Your Songs