Quit / open an app whenever audio output is changed on mac
Well, many thanks to CJK for the explanations! 

After some struggle with Keyboard Maestro (because of how Boom is made, a Boom2device appears in the sound output devices and never really changes, so the trigger from Keyboard Maestro doesn't work...) I tried user3439894's solution and got Eventscripts to work in less than 10 minutes!

The real plus with EventScripts is that you can choose which Bluetooth devices trigger the AppleScript.

Here's my set up:



With some scripts easy as:


Quit Boom:

quit application "Boom 2" 



or 


Activate Boom:

tell application "Finder"
    activate
    open application file "Boom 2.app" of folder "Applications" of startup disk
end tell



I also made another one that came in my mind to adjust the brightness of the screen after plugging the MBP to the power with brightness.