No one is good at keeping cognitive bias at bay. There are mitigation strategies, for those situations that don't allow outright elimination of cognitive biases (such as properly set up double blind tests). But the mitigation strategies are not proven effective (
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8397507/).
That being said, even if Amir has managed to get rid of all sources of cognitive biases, his subjective opinion would not magically become an objective opinion it would remain a "free of cognitive bias subjective opinion".
That's not a big issue for me as I trust Amir's subjective opinions to be better, in their general applications, than mine. In fact, I suffer from a cognitive bias that makes me treat Amir's subjective opinions as better than other subjective opinions. I recognize this, and can't overcome it
Still, it annoys me a bit that almost all headphones reviews end in an "EQ to subjective taste" conclusion. To remain true to the spirit of this site, a set of measurements after EQ could be provided. But yes, it would be time-consuming and probably lead to the same discussions that we have now...
Amir himself said
"I was very surprised at the effect of the little filter at 880 Hz which I added at the end. It really improved clarity of the headphone and seemed to reduce its distortion."
While I don't doubt Amir's honesty in his subjective opinion, that one looks like magic/random luck to me, given the measurements. It would be interesting to understand the why behind this tweak.