Is it due to the limit of headphone measurement, limit in EQ, or something else? How much of the so called speed of the driver (can our ear resolve below 1ms?), resolution, sound stage, etc, are just FR?
1990 has deeper earcup, two detachable cables, and two pairs of pads that sound different, I think it's a worthy upgrade to 880. I prefer the balance of 880/1990 over 600/6XX, still I need to eq down the treble peak at 7-8k using Solderdude's measurement. I tried to eq up HD600/6XX to sound...
https://www.audiocheck.net/soundtests_headphones.php
reverse polarity test is the center/twisted button, not sure if it will cause your described symptom though