It should be easy to review the DX90. The best evaluation I could do would be to suggest that you listen to one. That isn't what a review is about though. And there are times when people aren't interested in purchasing something, they would like to know something about the item, for interest sake.
For me, what makes reviewing the DX90 more difficult, besides restraints on my time, is that it does so many things differently. To my ear it doesn't fit a mold, nor is it a progression of a line or a step up or down from some other DAP. I find I need to explain new terms, or to define terms in a new way that to me, is how different the DX90 is. I hear music not as something canned and electronically presented but I hear music for the sake of music. Minor occurrences, the sound, in a recording, are presented as important to the role of what I hear as the main figure, or theme. As a story has an antagonist normally the counter to the protagonist with both built on so you can understand in more depth their characters and you have the foils, who are figures that provide the fill with much less is known about them and less is built on them but they add body to the story.
So it goes so much of the time with music I have heard. The main singer comes through, the instrument and you have all the supporting singers and instruments, sometimes taking their turns but you have the main drive, the nature of what the story is about. To have a story with all characters built, would be a joy but often too time consuming and sometimes unnecessary, unless it is a very simple outlay. The DX90 is able to pull from the recording, when well done, all the characters and lays them out to be heard in a dynamic space and time that creates and auditory image that belies the recording. It does not lay bare the music with intolerance, that drives a wedge between you and the music but brings the music to a closer and more emotionally satisfying place.
I often listen to Neil Young's live recordings that I have heard so many times before. With the DX90 I am not separated, I am there, and it is mentally tangible. Recently I have started listening to Greg Brown on his solo recordings. Brown's voice, which is so individual, drives the words sings. His guitar carries the line and you can feel the connection. On these recordings you can sense the space dynamic drive of individual sounds as they emanate from the space they occupy.
I have been using the Fostex TH900 and the Hifiman 560 quite a bit in conjunction with the DX90. I also use the ESW10 JPN. The Fostex and ESW 10 are rewired with Whiplash's new gold plated silver that I find that this gets the most out of the phones and gives me the reference I am after. The Hifiman 560 is used with either the stock cable or the copper cable that comes with the HE-6. These are the main phones I have used to this point.
I have tried several amps, my favorite the fi.Q with the DX90 but I don't find it necessary, even with the HE-560. The amp section of the DX90 is a marvel. I couldn't ask anything more out of a portable amp. By using separate amps, I was able to get a handle on what the dac section and amp section are doing and what each is doing is superb. This is a case where both compliment each other. All my listening impression that I present here are done with the DX90 in total, nothing added, nothing taken away.
I write here that the dynamics of the DX90 are the best I have heard from a DAP because they are personalized to the music, the performance and not generic. There is no compression of the sound, no bleeding of frequencies, no enhancement. The bass articulates, fluctuates to the recording. The mid frequencies give justice to the human voice or anything else that falls with their realm. Highs, to my ear are extended and without edge, glare or unnaturalness. In fact the entire sound is natural. With the S/N ratio of 119 and a crosstalk of 115 comes advantages. The DX90 has a silent acoustical background that builds the music, the placement, if it is close or far off sound is replicated and offstage sounds are realistic.
Take from my review what you can, what you will. For me the DX90 takes the recording you hand it and gives you back music and that is an experience I can live with every day.