It would be somewhat surprising if dominated by third harmonic given the advantage of a sealed box where no air is pushed out of slots, but if it's from the loss of spl due to cardioide interference and necessary EQ to up the level in the 100-300 hz area it could very well show much the same.
I'm painfully aware that far-field in-room measurements can't be trusted for distortion accuracy, but at least here's a measurement I found of Kii in the listening position after Audiolense adjusted frequency response to compensate for SBIR dip in the 200-300 range. Probably about 10 dB boost in that area, 3,5 meter distance and one speaker playing.
View attachment 167876
It'll be interesting indeed to see complete set of measurements
SBIR dip?? Are the speakers mounted to the ceiling or something?