How do I mute my monitors while leaving headphones on?
you are in the correct track: a small mixer would do the trick. Basically you have the pfl button that would get the specific channel(s)/souce(s) selected to your headhones jack (monitors) without modifying at all your master (and viceversa). Another option would be something like the Mackie Big Knob, made specifically for what you want, but in this case you can only select to monitor a source, not individual channels).