Is there a huge difference between d7200 and d9200 in spatial 3d sound effects? Or a minor difference?
IMHO they are similar in sound stage, which is expected, since the acoustic environment is almost the same.
But when you change the ear pads to have larger opening, the sound stage scales up considerably. I'd say the optimal sounding ear pads for the Denons have similar shape and materials as now (good job, Denon), but ~5 mm more internal diameter, ~5+ mm more external diameter, and ~20-30% less internal foam mass. That means shallower pads than now. If someone needs deeper pads, need to tune other parameters. Such pad options don't exist on the market. Beware that the FR is changed by that, for somewhat more recessed mids and deeper (but not more prominent) low bass. The closest would be the D5200 ear pads on the D9200. Anyway, the stock pads are pretty good to start with, even if they compromise sound stage for better FR measurements.
As said so many times, the main difference between them is the drivers, which is IMHO far better in the D9200: stronger magnet, slightly different diaphragm and suspension. The end result is better mids+bass resolution, deeper positioned low bass, better microdynamics, and also the treble nonlinearities, similarly to the TH900 drivers compromises. IMHO the D9200 are well worth their price difference from the D7200, but the D7200 (especially if they lately changed the drivers indeed) are not far behind, with more midbass, less low bass and far more linear treble, making them a phenomenal value in closed headphones (and perhaps the most linear measuring).
I think the next possible wave of "good sound" might come when adding to these designs Bluetooth/USB/DAC, but using DSP to tackle the driver issues with FIR/noise shaping, avoiding phase issues, as opposed to IIR filters / EQ. But the latter can do also good. The Dali IO-12 and T+A Solitaire T (and price-wise the B&W PX7 s2, too) are good examples for bio-cellulose / paper drivers run via DAC/DSP, which even with lower class drivers than these Denons manage to sound pretty good, much above their weight. The carbon drivers in B&W PX8 and Sony XM5 could be better as well, but they seemingly intentionally chose a bad, bass heavy voicing (suitable for outdoors and air planes, which are their primary intended environments, it seems). The Denons are only for indoor use. But I don't know if it's worth for the manufacturers to add BT/USB/DAC/DSP FIR capabilities to these existing designs, especially with the HW needed for FIR filtering, as the price would likely be prohibitive. The good news is that all this can be done today in a desktop headphone system (or active speakers), only not portable yet, AFAIK.