Had the incredible honor of attending Easter mass here. Just a beautiful ceremony at one of the most incredible churches in the world. I also had the great privilege of hearing the massive organ played. I felt that going to the mass was a fair substitute for the tour.
I also recommend going at a time when you can hear the bells being rung. If you are looking from outside, it is truly incredible to watch the effort it takes to ring the bells.
This is another must-visit on any trip to Montreal.