This and That:
In my opinion, the DX90 is the best digital audio player that was ever made. I am especially happy that I bought it, since the DX50 that I bought before it was a big disappointment for me (poor hiss performance, small measurable sub-bass roll-off).
First the bad things - its battery life is pretty poor and its screen resolution is rather grainy as well (only 320 x 240 pixels at 2.4 inches), and the album cover that's displayed at an even smaller size in the upper left corner is barely distinguishable. In addition, it still lacks a track counter on the playing screen.
Other than those things, it is quite perfect.
While the UI isn't the most modern looking, everything is easy to use and navigation is very intuitive. The capacitive touchscreen is responsive and the player never feels slow. The only time when the player feels slow is when one performs a database update.
The folder browser works just as it should and except for the missing track counter, the playback screen doesn't really lack anything.
In terms of visual design, it's almost identical to the DX50 with only a few subtle differences.
It comes with a silicone case, at least one screen protector (I don't really remember it that exactly), a highly questionable burn-in adapter and a coaxial adapter.
In terms of storage, it features an internal memory of 8 GB and accepts micro SD cards up to 2 TB.
It's got a three-stage analogue gain switch.
Gapless playback works perfectly with FLAC files.
Volume Control:
… is simply perfect. 256 steps with a scaling of 0.5 dB per step over the entire range. Therefore listening very, very quietly even with the most extreme of extremely sensitive in-ears is easily possible.
Sound:
The DX90's internal design is fully symmetrical (real dual-mono) but it doesn't have a balanced output (that I don't need anyway). So why this extra effort? To achieve even slightly better measured performance compared to a non-symmetrical stereo implementation of the DAC and Amp section. Imo, even though the result depends of course on the implementation of the components and not what was used and how it was used, this shows that iBasso really aimed for creating the best they could, and they did (, as loaded and unloaded measurements performed by other people on precise laboratory-grade recording equipment proved). In that way, they really created a true flagship masterpiece and released it at a time when they weren't as well known as they are now, and priced it very affordably in relation to the performance.
Frequency Response (Unloaded, Digital Filters):
Output Impedance (Fast Roll-Off Filter, Loaded with Ultimate Ears Triple.Fi 10):
The calculated output impedance based on the deviation is only around 0.2 Ohms, which is perfectly low for all multi-BA in-ears, even the very critically reacting ones.
Hiss Performance:
Hiss performance using very, near-extremely sensitive in-ears, such as my Shure SE846, Ostry KC06A and Pai Audio MR3, is perfect. No audible hiss (quiet environment, "empty" audio files) (except for the Ostry that, with high concentration, only shows a bare hint of hiss that's almost completely indistinguishable from my bloodstream's noise; but even when playing music so quietly that it is barely audible, there's no hint of hiss anymore).
Using my Campfire Audio Andromeda that is even more extremely sensitive, very mild, almost inaudible hiss is present. So the DX90 is "only" "almost perfectly perfect" in this regard (out of the devices whose output impedance is below 0.5 Ohms, it's only beat by my RME ADI-2 DAC and Leckerton UHA-6S.MKII that are fully quiet even with my extremely sensitive Andromeda), but it's the only digital audio player with an output impedance of less than 0.5 Ohms that I know that is this close to utter perfection (devices like the Cowon Plenue 2, Luxury & Precison L3/L3 Pro and FiiO M7 are perfectly quiet but have an output impedance that is higher than 0.5 Ohms). The DX200, DX150 and even DX220 are, for example, hissier compared to the DX90. The same goes for the FiiO Q5 with the IEM-oriented AM1 module. My Chord Electronics Mojo also outputs the clearly louder hiss in comparison. And even when used as a pure amplifier, the FiiO Q1 MkII is ever so slightly hissier. The list could go on and on.
So indeed, the DX90 is basically perfect in terms of hiss performance, outperforming the vast majority of other audio devices. By the way, the gain setting and volume setting don't have an influence on the audible hiss performance.
Subjective Sound Perception:
Clean, neutral, transparent.
I'm on the objective side in terms of audio and don't care about
subjective blah-blah nonsense. Only controlled testing environment.
It may appear subjectively a tad more "aggressive" in terms of cymbal rendering and a bit "wider than deep" sounding. It's not "night and day" by any means but just a nuance and irrelevant to me, and nothing I care about or notice during casual everyday or even during critical listening. And it's only something that I can reproducibly hear using sensitive in-ears while my DX90 sounds exactly just like any other well-designed device when I'm using it with my full-sized headphones or less sensitive in-ears.
It may appear subjectively a tad more "aggressive" in terms of cymbal rendering and a bit "wider than deep" sounding. It's not "night and day" by any means but just a nuance and irrelevant to me, and nothing I care about or notice during casual everyday or even during critical listening. And it's only something that I can reproducibly hear using sensitive in-ears while my DX90 sounds exactly just like any other well-designed device when I'm using it with my full-sized headphones or less sensitive in-ears.
Conclusion:
Perfect volume control, perfect output impedance, (almost fully) perfect hiss performance; otherwise excellent measurements as well. Simply a truly great device that has raised the bar very high.