Any way to reset volume level after unplugging headphones?
You can use a third-party app like Keyboard Maestro or EventScripts to trigger a low volume setting script whenever you plug your headphone!

For exemple, with EventScript:


Click on Open Folder from EventScripts menu in order to open the scripts folder.
Add the script you want to be triggered, in your case something like do shell script "osascript -e 'set volume 1'" (note that set volume function works from 0 to 7, 0 being mute and 7 maximum volume).
Click on Choose Script from EventScripts menu and select the script to be used.
Add the event Headphone connected.


See my own setup as exemple to quit or launch Boom whenever headphone or Bluetooth device is found: