Your fine as is! I only got the MX a few years later. You can get third party modules from Fir audio as well that are cheaper and do something similar, but especially with your preference, the m15 and m20 will be fine

You've picked well. I still adore mine like 6 years later.

Edit: I see you're concerned about bass. Don't be. Because of the custom seal, you'll get bass rumble in your gut from these puppies, while limiting fatigue due to the apex modules. And speaking of, you can also adjust the freq response slightly with different modules, both first or third party, so you can tweak things around just in case. I've noticed in more recent years I prefer a slightly leaner tuning, the bass is still there, but the overall balance for all sorts of genres is better to my ears (I use the MX)

Yes! East side indeed.

Half of Oahu did this too. The whole west east side of the island collapsed overnight thousands of years ago. Can see debris halfway to North America

Huh TIL. Yeah that's it precisely. Also still have my HD6XX around too and they are great

Freq response is like the color tonality of your TV, resolution is how many pixels it has. I often wonder why this is so hard for peeps here to understand - there's even charts to measure "resolution" in terms of accuracy of various frequencies over time: waterfall charts. In those you can see which freqs have more decay. High resolution headphones have shorter decays. Planars have short bass decays for instance. To simplify: Freq response is a 2D snapshot of a 3D scene. Resolution requires that snapshot to be measured over time with how accurately it moves air precisely within that freq range.

Much of this "freq response is everything" I feel is wrapped up in classism (these $1k "high resolution" headphones can't possibly be worth it, especially because I can't afford it) and confirmation bias than anything else.