Glad to see this topic of optimizing multiple subwoofers being discussed.
I've written a freeware program called
Multi-Sub Optimizer (MSO) that automates many of the tasks mentioned in this article, and takes what you can do further than what is explored in the article. It supports the miniDSP products using IIR filters, including the 2x4, and has the ability to directly paste the filter coefficients into the "Advanced Biquad Programming" interface of the miniDSP software.
Until very recently, MSO has required an XLR or other analog microphone, along with the loopback timing reference of Room EQ Wizard (REW), because time-synchronized measurements are required. Earlier this week, John Mulcahy, the REW author, released REW 5.15 beta 3, which introduces the "acoustic timing reference" feature, allowing MSO to be used with USB mics. Discussion of the acoustic timing reference feature can be found
here.
The MSO program can be downloaded
here. There is an
extensive tutorial available. Working through the tutorial requires the
example files and projects.
I've also described the algorithm in some detail in the
MSO reddit thread.