The absolute best app for the Mac in my opinion is menuBus (
https://www.menubus.audio).
menuBus is a 'must have' for giving you complete control over all audio on your Mac across all apps (iTunes, Skype, Safari, YouTube, ...). menuBus runs seamlessly in the menu bar acting as a 'master bus' for all audio on your system. Its surprisingly simple interface allows you to drag and drop audio plugins in to the 'bus' for performing systemwide audio equalization, on-demand recording, multi-channel audio splitting, speaker and headphone calibration, you name it. Originally intended for professional use in recording studios and for music creation the app is is highly optimized, bit perfect, glitch free, and rock solid.
To this end, the developer Nathan Tiddy who happens to be a really cool guy (a goat farmer in New Zealand) actually listens to the user base and is constantly improving the app. On his website he provides examples of using it with free plug-ins to correct the frequency response of AirPods to make them 'studio reference' flat and is always super responsive to questions by users on his Facebook group page where users even have a vote for the next great feature he focuses on.
While even the free version has plenty of features for the majority of users to enjoy for only $20 the 'Pro' version is well worth it for the ability to save off unlimited presets activated by user-defined hotkeys.
This is an app that seriously Apple should just include in macOs.