The AD700X seems to have a somewhat ... weird FR:
https://github.com/jaakkopasanen/AutoEq/tree/master/results/crinacle/gras_43ag-7_harman_over-ear_2018/Audio-Technica ATH-AD700X
Your current Sonys track the Harman target a lot closer. The K702 will be more treble-forward compared to the Sony.
If you happen to have too much money, you could try to double down on that earspeaker-design and look for some Stax.
If you want to hear for my take on it: Never much cared for the 1840.
As far as sound is concerned, I never understood why it should cost 500€. Did a comparison recently with HD 600, K702 and DT 880, can't say that the 1840 really gives you anything more than those. That goes for many headphones in that ~500€/$ price class, they don't tend to be all that different from the entry level HiFi-gear. But at 200€/$ that is a completely different story of course.
What I couldn't get over though are three things concerning the construction:
- thin headband pads
- fairly small earcups (if they fit, they are comfortable though)
- tiny, fragile MMCX connectors wich are completely misplaced on an over-ear headphone, so be careful with it