Nice. It's always great to see confirmation of my results in a different room.
Here's my go at the same system in my room, showing naked response, response after the SEQSS signal shaper and at the LP, directly compared to yours:
The SSD version of the SEQSS has a notch filter and bi-quads to remedy the inductance peak (the naked response trace is smoothed showing a tempered peak). That corresponds with your raw peak which is exaggerated by your room (yeah, ain't it always the way?).
My result is with the mains, which were used to tweak the crossover region with cross point at 100 Hz and I suspect that your results (if you had only the subs running) will be similar if you tweak the distance (phase) and cross point first, then apply post EQ.
Anyway, as I said since the first run through with the SSD-based Blackbirds, it's a heckuva system for the cash outlay. I found no complaints from the panel of musicians I use for subjective eval, and they are "brutally frank".
Nice work, great looking industrial design and execution. Looking forward to your comments with the ULF heavy hitters (WOTW, HTTYD, Tron, etc.) Please post pics once you've gotten the top plates installed?