Thanks for this kingkorg! I've never thought those factors would affect the frequency responseHere's the effect of a headgrille. U87 is designed to have a transparent one.
https://groupdiy.com/threads/wa-47.79979/post-1047689
There's also this, position of the capsule is also important.
Post in thread 'Effect of capsule height in head basket' https://groupdiy.com/threads/effect-of-capsule-height-in-head-basket.84530/post-1097866