menuBUS is a lightweight menu bar application designed to capture all system audio, host realtime AudioUnit effects, and pass the processed audio to your choice of output. Simply launch menuBUS, apply the Audio Unit processing desired (room/headphone correction EQ, for example), and tweak the plugin parameters. There are unlimited plugin slots, easily rearranged via drag-and-drop. The app remembers your previous settings on reboot and can save presets. You can even add custom keyboard shortcuts to adjust volume or switch presets.
menuBUS is currently under development and in beta. It is due to be released in March for $39. Beta participants are being promised a discount upon release. It requires macOS 10.10 or later.