Just read this thread and downloaded Soundsource.
I cannot believe I didn't research this years ago.
I LOVE this program.
If you have a Mac Pro you can have your external speakers plugged in the rear and if you plug headphones in the front headphone port, the sound will automatically switch to your headphones!
Again, how many friggin' years did I spend opening the sound panel in system prefs to switch jacks? What a dope.