I had read somewhere that while it was aiming for diffuse field EQ, there was a difference between diffuse field equalized and loudness diffuse field equalized, and beyerdynamic didn’t aim for the latter. I don’t know if this accounts for the difference in 8.5khz peak, between a beyerdynamic and a sennheiser, but that’s certainly one of the most striking differences.
Although this post also talks about the Beyer peak as a result of the ‘crinkle membrane’, so it could just be that the rest of the frequency response was tuned for the diffuse field target, and that’s just what happened at 8.5khz haha.