After a while with the DX90 I can confidently report that I am hearing some things out of the Dunus that I haven't heard before out of any combo. The DAC and amp in this is absolutely amazing.
The bass is more or less the same in quantity as the DX50, both are quite neutral in the bass area. I do, however, get an impression that the bass is a little heavy because the DX90 has better sub-bass performance and that makes the sound seem a little on the thick side. Now that I think about it, it's probably the DX50 that lacks a little bit of sub-bass rather than the DX90 having a boosted sub-bass region. The bass is slightly more detailed and has better clarity.
The midrange is ever so slightly on the warm side, but that might just be me. The DX50 sounds a bit peaky in the upper midrange/lower treble area, which results in it making some headphones sound sibilant or lifting a veil on darker headphones. The DX90 is a little closer to neutral but it leans a tiny bit to the warm side, but this is likely to change with burn in.
The treble is cleaner and less grainy than the DX50. It feels a little bit more controlled and less fatiguing than the very slightly bright DX50. This makes the treble less fatiguing and less prone to sibilance. For me, this is a big plus since the DX50 could get a little tiring to listen to for long periods of time. It does very well with the bright Dunu DN-1000s. Let's see how it will turn out after some burn in.
The soundstage along with the imaging is perhaps the largest upgrade of all. The DX90 makes the DX50's soundstage narrow and shallow. This is very evident throughout the entire time, but the occasional cymbal on the sides of the stage make it even more obvious. The depth and height of the soundstage has also been improved quite a bit, but not as much as the width.
The imaging is simply stunning. The pinpoint imaging and the fact that the DX90 can make it seem like there are layers in the music is not there in the DX50 or even the X5 that I tested.
The separation and detail is also something that I have to mention. It has revealed minute details in the music that I have never heard with my Dunus and it makes some details in the background much more noticeable. This is absolutely incredible because the DX50 is already a very detailed player, besting the X3 (IMO) in this regard.
The DX90 has absolutely blown my mind. It sounds so damn good and it costs a relatively small sum of $420 compared to other players. It is also mentioning the build is extremely solid: the custom jacks and the buttons all feel like they are built to last. The touchscreen is a little too sensitive, but iBasso will most likely correct this problem in the near future. It is hard to imagine something besting at this price. iBasso have really outdone themselves.