I agree with most of your findings. 560S is so bright I thought they were broken and got another pair just to confirm they're really bright. It's mostly about the imbalance between lacking fullness in the lower mids/upper bass vs emphasized lower treble while also being a bit peaky in the upper treble. I actually find 58X to sound more neutral. But with 58X, pads are very important. Out of the box they are even a bit sibilant. When pads are worn they're too muffled and that's when it gets muddy. Here is the FR of 560S against my target and here is 58X.