Great review! I remember there was a website that documented frequency curves of different headphones. When I was doing some research to get my current pair of semi-professional monitors, I used the website to pinpoint the ones with reasonably flat sound. Apparently there are a lot of review websites that post their frequency response charts, so they're a great tool if you haven't already used them.
One thing about jumping headphones is that you will get used to your curves even if they're bad, as long as it's your only pair. So listening in to bad curves will normalize them in your brain and sometimes give a perception of worse sound when you get on something flatter. I experienced this when I switched from my moderately bass boosted headphones from some Chinese brand to a comparatively flatter curve of Audiotechnica's ATH-M40x. It felt like a downgrade for a while.
Anyways, I'm sure you know all this already, but I thought it might be helpful context for others. Would be nice if you could dial it similar to your main monitors with an EQ and see if you can overlay the resulting curve with a chart of your DT1990s. I think it would be a helpful point of reference for other buyers.