You got me!
The resonance at 550Hz has a wavelength of 0.6m, which makes influences of the speaker cabinet due to the wavelength unlikely.
For a surround resonance the interference at 550Hz is too low, should rather be in the range around 1kHz. Assume that the chassis is a (modified?)
SB17NBAC35-4. There is a review (Hobby Hifi 2018-6) of this loudspeaker chassis. The range around 500Hz is completely free of interference (FR + Imp).
Maybe it has something to do with the passive diaphragm, but then I would expect a resonance in the impedance frequency response too - the same would apply to a defective woofer.
On the other hand, relatively low frequency resonances also occurred in other loudspeaker measurements - which were also unusually pronounced.
View attachment 60360
During the measurement of
@hardisj the interference was not visible.
View attachment 60361
Therefore I would not exclude that the resonance is caused by the measuring device itself or the environment.