The MDR-1A has a THD averaging about 1 percent at 90 dB which is very loud. It’s true that it distorts a little more than that at 100dB, but by that point it’d be a very unpleasant listen and as you mentioned, you don’t like to listen to things at loud volumes.
I think the MDR-1A is very good, and feel like you returned a keeper, based on what you’ve said about your own tastes.
The Vmoda M100 has a comical bass boost, something like 10dB over neutral, and it is not at all a good mixing headphone, in my opinion. I also think the included ear pads are sub-par, comfort-wise, and the XL pads are basically a requirement, which always turned me off buying a pair of my own.
If you keep chasing numbers, you won’t be happy with anything! :)
Equal loudness curves are hard to achieve on headphones because of the nature of the physics of their tiny drivers. Most headphones target the Harman response, which is designed to mimic the sound of flat speakers in a room. I personally really like the older Diffuse Field curve, which had a similar aim, but was made in an era with less precise measurement equipment.
Some headphones use electronic processing to try and boost bass at lower volumes, giving the effect of equalizing loudness. The Bose QC 35 is famous for doing that, but more pricey than most of the other headphones you’ve mentioned.
Generally for sound work, I like to have a pair that will highlight recording flaws, and then it’s good to listen on a Harman-target pair, or a pair of flat monitor speakers.
Have you considered using monitors for mixing instead? Or studio stalwarts like the M50X/M40X/7506? They’ve become standards for a reason!