AKG K702 is pretty smooth according to Crinacle's measurement, and this measurement I'm displaying here is not an average, it's just one measurement, so it doesn't have the smoothing effect that Oratory's graphs exhibit (by virtue that Oratory's graphs are an average of many measurements):
View attachment 113688
So while your worn pad DT990 graph might be quite smooth, you probably can't claim it's the smoothest frequency response because it's an average of many measurements. There's zero smoothing applied in the above K702 graph, you get to choose how much smoothing is applied in his graphing tool, and I set it to zero, so that's the raw measurement right there, and just one measurement. It's not smooth north of 10kHz, but Oratory's approach of moving the headphone around a little and averaging many measurements would smooth out that 10kHz+ area to some degree.
I just think it's a bit dangerous of you to say that DT990 has the smoothest frequency response, hence this post of mine.
EDIT: your graph also has a massive spread on the y-axis, making the response look a lot smoother than it actually is, unlike the zoomed in y-axis (low spread) of my Crinacle graph I'm showing in my post here). Your graph has a 60dB spread on the y-axis whereas my Crinacle graph above is zoomed in and has only 29dB spread - it's zoomed in twice as much.