I too have long subscribed to the multiple sub theory, but two X13's were just too much for my theater. Fully treated room is only 2100ft3, and i could not back these monsters off enough so they would not dominate the room. At one point during an explosive action scene using DD Master 7.2, i thought maybe i just made the room bigger !
Im sending one of them back - one is plenty for me !
I will however augment the X13 with one of my smaller subs because i just have to be able to say ive got two !!

That’s good feedback, I only have one X-13 and have thought about adding another for the many benefits of having multiple subs. I found that even with only one (level matched using REW) I still had to dial back the LFE trims in the LFE menu, just wondering if you also adjusted those?

I started to do that, but i stopped myself with the logic that if i have to dial them back more than a click or two, then two subs is too much to begin with. At $1100 each (december discount), i couldnt see not utilizing their full potential.
Listening to multichannel or stereo music, which rarely got the X13's even started (probably zero content below 40Hz) was a giant waste of money. The first Master Audio 7.2 Bluray i played had a constant overpowering hum of bass energy in normal scenes that have an underlying sense of shits-going-to-happen soon ! It made the movie unwatchable, but when that storm did hit, it about blew the walls out of my room !! The same scenes with just one X13 was far more enjoyable and the big hits were still major impressive, whilst saving my wall studs !
I worked with the two X13s for a few days but the room is just too small. These things are strong !

It sounds like you had a bass node near your listening position but I would agree that two x13 is too much for 2100 ft3. Two M8s would probably have been a good choice.

Update: This X13 is incredible. Ive had a bunch of so called subwoofers over the years, including some ive made myself, but ive never really experienced true sub bass until now.
Ive left it with one port open, set to EQ1, for the lowest response curve. If the graphs ive seen are accurate, response is still very flat, but this is not its THX mode.
I watched "Edge of tomorrow" last night. There is a sound track at the very start of this movie that goes lower than ive heard anywhere before. Reviewers say it approaches 10Hz.
The X13 reproduced this at stunning pressure with no audible distortion, although it was scary strong and made me jump for the remote !
My son, two floors up, came running down to the basement thinking there was an earthquake. I swear the single foam plug is working its way out of the port !