Ok I have come to conclude this quest for the perfect settings. First let me reveal what makes 2.2.0 L0 special: While all other firmwares invert the sounds, this one does it right. Test it on a song you know well (where for instance the singer sings close to you and the band is further from him) and see what I mean. Or just take my word for it which is much easier
Still the resulting sound did not feel to me like the "reference" sound, and by that I mean the result as the sound mixers/engineers intended it to be. So I applied EQ and I believe that the case is closed now.
So to sum up the settings:
1) Install 2.2.0 L0 firmware from Lurker's page
2) Create Custom EQ 0, 1, 2, 0, -2, -2, 0, 2, 1, 0 and turn it ON
3) Use LOW gain
4) Use Sharp Roll-Off
That's it !!!
After that the "chain" is yours to fix/adjust, by trying headphones, amplifiers etc. But up to that point it is my belief that you get the closest to studio-output sound. I invite Paul from Ibasso to try this and tell me his opinion. After all it is kinda sad that, while they managed to build such a wonderful hardware, they couldn't let it shine due to software. (they even admitted that the sound signature changed from firmware to firmware despite their intentions)
(One note regarding people that talk batteries. I do not believe that a battery other than Samsung's that is "feeding" the dx90 with the same voltage should make a difference in sound at all because I don't find any sense in that happening)