Despite your doubts, I suspect that the bump is coming from the woofer cone-breakup.
This also corresponds very well with the increased third order harmonic distortion HD3 at 1.5-1.8kHz.
With cone-breakup the bump can be 10-15dB and depending on the driver it can even increase under angle.
That's probably the reason why the bump under angle hardly gets weaker.
This bump must be specially treated during crossover to avoid negative effects.
Don't say that this chassis was used, but the Dayton DSA175 is a good example of such a case:
View attachment 52967
The increased HD3, caused by the heavy cone-breakup, is the reason why metal cone chassis have such a bad reputation. All too often, the crossover is too high, causing high HD3 - as in this case.
https://www.parts-express.com/dayto...designer-series-aluminum-cone-woofer--295-528
https://audioxpress.com/article/test-bench-dayton-audio-dsa175-designer-series-6-5-woofer