He talks about how two subs can be used to cancel length modes. Nyal is the first person to introduce me to this concept a few years ago. Unfortunately, it's still not a well understood method for reducing room modes at a single position. IMO, it's the best way to setup subs for a single listener. Of course, he knows a lot more than what's stated in this paper. There's not sufficient detail in this paper to describe a method for integrating 2 subs to destructively drive the length modes in this paper.
He also mentions Welti, et al. The Welti method is not designed to destructively drive modes. It's much better to describe the Welti method for setting up 4 subs as a room mode distribution method which results in lower mode amplitude and a wider bass sweet spot. IMO, the Welti 4 sub method is better suited for the home theater which needs a much wider bass sweet spot.
I'll see Nyal in a couple of weeks. Maybe I can get him to listen to my system again.