In the recent firmwares from the DX90, I feel that the sound has actually become more forward with DX90, which was a good thing to me as I felt that the vocals were more distant because its slightly more recessed, not because the DX90 has amazing sense of depth.
Comparing the two, the DX90 sound more in your head, while the D14 presents the sound as being slightly more in front of you than the DX90. So I would say the D14 has a better sense of depth as well in terms of soundstage.
Honestly, if you A/B the two, you can tell the difference between the two, and that the D14 sounds a bit cleaner and better. But when you really spend time with one, then switch to the other, it becomes really apparent that the D14's sound is quite a bit more expansive, has a much more natural sound with better separation as well.
Hope this helps! I don't want to undermine the DX90 as I love the DX90 and use it when I need something truly portable, but its been almost two years, and iBasso has gotten better at what they do!