Connect to a small mixer and pull the faders down. My monitors also have a remote mute function so either way I can turn them down.

Hello Darren - David's nailed it, a mixer is useful in many ways. It puzzles me why monitor makers hide the volume control round the back, where I bet they're unreachable by most people. Why not on the front, for Pete's sake?

Most devices cut off the speakers if headphones are plugged in, but if Montage can do that they've hidden it away well ...

I wouldn't recommend this as a general solution to your question - but using Assignable L&R as outputs - since they can be volume adjusted independent of the earphones - may work for you. Assignable outputs are not functionally equivalent to Main L&R - but if you are just going to use your speakers as a general reference you may not mind the loss of features when using assignable L&R. Some possible issues: assign L&R does not include the master/system effects and assign L&R (assigning PARTs to output to Assign L&R) requires configuration - which may become tedious depending on how many performances you're working with.

Just wanted to offer another possibility and explain some of the caveats.