Frequency response specs on headphones are total BS. They mean nothing and are
putthere by the marketing department.

Completely agree.

I used to use a pair of Beyer DT109's because I was suckered in by the
implication of quality through their inflated price. Truth was they
were poorly built and had appalling sound quality. After a few years
the plastic ear muffs starting cracking because the wrong plastic was
used, the electrical connections broke but at least the sound quality
remained as original, though quite awful!