As I state above I am no expert and I am sure that there is a more tecnical way of doing this however..........................
When at the JL Audio Facility tour a few years back (that was just an awesome visit
) it was very soon after the failing of M&K. JL was lucky enough to grab one of M&K's best sound engineers and this specific question was asked His (I am sorry as I do not remember his name maybe one of the guys who was present can bail me out on this one) response was...........
" The best way to do it is to position your head between you sub and one of your mains, which ever one it is closest to. Then play music with a pretty good beat to it and adjust the phase until your mind is hearing the beats at EXACTLY the same time from both the sub and the mains. Do not get caught up on what the phase setting ends up at as this does not matter. Think of phase as "Timing" if you will."
I hope this helps and I hope if there is better information out there that it comes your way. I have however done this same setting with my Super-Cube Reference and I have to say IT WORKS.
Now you have to tell us all of your impressions of the Trinity......... PLEASE.
Enjoy your sub it is one of the better ones out there.