SpeakerAmp 2.6

SpeakerAmp lets you amplify, equalize, and boost your audio with 3-D surround. It increases system-wide volume up to 9x louder beyond the max Mac normal level.

The tool works with any output device. You can set the volume range for each device individually. For instance, you can set lower values for headphones and higher levels for a soundbar. The app changes the settings automatically upon output device selection.
Similarly configurable is the active equalizer profile.