@Rskmd – you are at a resolution that you cannot get accurately outside of a Klippel. At below 100Hz, you need to do ground measurements. Please see the documents related to sub testing here:
https://www.erinsaudiocorner.com/subwoofer_testing/cea-2010_subwoofer_testing/
https://www.soundandvision.com/content/cea-2010-good-baseline-bass
https://www.audioholics.com/loudspeaker-design/2010-subwoofer-shootout
https://forums.audioholics.com/forums/threads/erins-cea-2010-subwoofer-testing-master-thread.119322/ (discussing his use of CEA 2010 B instead of A)
https://audioxpress.com/article/cta-2010-a-better-way-to-measure-subwoofers (an argument for CTA, formerly CEA)
https://www.audioholics.com/loudspeaker-design/subwoofer-measurement-data
And Erin addressed what he used, why, and he also compared results to the near field scanner he owns.
So, part of the problem for under 100Hz is resolution, part is testing methodology. So, I cannot find the CEA text right now or I would send it to you.
https://shop.cta.tech/products/standard-method-of-measurement-for-subwoofers (free paper on testing, revised for 2020, audioexpress explains why the name change; ANSI/CTA-2010-B R-2020)
https://webstore.ansi.org/Standards/CEA/CEA20102014ANSI
https://webstore.ansi.org/Standards/CEA/CEA20102012ANSI
https://webstore.ansi.org/Standards/CEA/CEA20102006ANSI
3120-ANSI-CTA-2010-B-R-2020-Final.pdf
