I followed the code path when you change the cabinet type, and saw it write some values to the DSP based on a 2D array of doubles, one for each cabinet, each with 41 values, and it was processing them 5 at a time. Looking at the values, they were all in the range -2 to 2, and were very reminiscent of biquad filters I had learned about in another project (https://mforney.org/blog/2025-06-06-babyface-midi-protocol.h...) which was still pretty fresh in my mind.

I tried plotting them, and I got something that looked right when I inverted the denominator coefficients. I guess this is fairly standard practice because then the difference equation is all positive sums and it can be implemented with a bunch of multiply-accumulates.

However there were still some discrepancies in overall gain between different types (most lined up, but a couple did not). I saw another array of integers indexed by the cabinet type that had negative values, most with -23 but a couple with -12, which I figured must be a decibel gain correction. It was only after accounting for that and seeing the final graph in the post where everything lined up and looked plausible that I was pretty sure I had it right.

So, mostly just general familiarity with digital EQ filters and a bit of luck.