I purchased the Sony NW-ZX2 in October, 2015 as I already own the Astell&Kern AK120, I was looking for a Hi-Res player with better audio quality, longer battery life and a better user interface. One of the first things you notice about the ZX2 is it's weight and size, it is hefty and solid and larger than most cell phones though most of the top surface is devoted to screen space.
The headphones I used were the Sennheiser IE800, AKG 3003i, the JH Audio 16 Pro, and the Sennheiser HD800 alone and amped through a Schiit Lyr2. The reason I reference the headphones used is because to fully appreciate all the audible qualities of this and other Hi-Res players, you really should use headphones capable of reproducing them. The music I listened to was either in lossless FLAC or DSD format, I don't have any mp3 files.
I started "burning in" the unit as recommended by Sony by having it powered by a 2.0A USB charger for the first 24 hours while playing through two different headphones, the Audio-Technica ATH-M50 Impedance: 38 ohms and the Sennheiser HD800, more difficult to drive with an impedance of 300 ohms. After the first day I decided to pull the plug and play it on battery power alone to see how long the batteries actually last. I manually turned off: WIFI, Bluetooth and NFC and just let it play continuously through the more than 3,000 songs on my 128 GB micro SDXC card. It played on battery power continuously for one day, 11 hours, 14 minutes with 15% battery power remaining, (see photo of screen) which is a pretty amazing 35+ hours on battery alone with the volume set at about 80%! I plugged it in again to charge it while still playing which took about 4 hours and then repeated the same thing, played it on battery alone, again with the two headphones mentioned above which BTW played so loudly, I could hear them in the next room. This time I let it play one day, 14 hours and 50 minutes, nearly 39 hours with the remaining battery life reported by Android (under Battery) as 8%. To further preserve battery life remember to shut the unit down when not in use. The first time it is powered up after you add songs to it's memory it can take several minutes to read and create a new database but once this is done, it takes about 10 to 15 seconds.
BTW, I disabled the "Sound Adjustment" application in the Android: Settings/Applications/ALL - then scroll to the bottom and press disable. This removes any sound embellishment programs that Sony has added which to my ears, sounds much better, far better SQ and has the added benefit of increasing the volume output of the unit significantly so it was actually able to drive my HD800's and I had to reduce the volume though for optimal appreciation of the HD800's I recommend a good Headphone amp!
So how does the ZX2 sound - marvelous! I was actually quite surprised as I thought the AK 120 was extraordinary. The dynamic range when playing Aaron Copland's: Rodeo in 96/24 (Detroit Symphony) was amazing, the background is completely silent, no hiss, no noise using any of the headphones mentioned, the only sounds you hear between movements are the sounds of the orchestra moving in their chairs, turning pages or placing their bows on the strings of their instruments. Classical music can be quite revealing and this selection is known for it's quiet sections and it's loud crescendos from the horns, cymbals and the bass drums. The ZX2 is spot on faithfully reproducing every instrument without blurring any of the finer details and the soundstage is enormous especially with the HD800's and the AKG 3003i's. As I have heard this piece live several times in different concert halls, I know what it should sound like if you were actually there. Symphonic music is quite a challenge for most players and headphones because so many different instruments are used simultaneously and so the ability to hear them as distinct rather than smeared together is a good test of their accuracy (when a good recording is used).
Next I listened to another live album that is excellently recorded, Alison Krauss & Union Station Live in Louisville and again, the sound was immersive, the sound stage spacious, when the audience claps you can hear every individual, nothing is missing and nothing is added. Alison's voice is clear and nuanced without undue sibilance, the instruments are distinct, you can hear their fingers move across the strings, even the subtle harmonics at the end of a song are audible.
I would like to point out how well the ZX2 handles bass, it seems to be able to go one octave deeper than the AK120 and the control is excellent, the bass is quick and at no time does it sound boosted or enhanced. When the album has deep bass as on the Bill Evans "I Will Say Goodbye" where it's played on a double bass by Eddie Gomez, it's musical, nuanced and there in all it's depth. If it's there you'll hear it and if it's not you won't and that's how it should be.
Some people have complained that the Android interface isn't as responsive as it is on their phones and they are correct to some extent. At times, you might have to press on an icon more than once for the player to respond which isn't a big issue for me as I bought it for the sound quality, not for games, video's or surfing. I found that it you go to Settings > Apps > All, there are several apps you can disable which you probably won't ever use and doing so increases it's responsiveness to touch input as there are fewer apps running in the background utilizing memory and processor resources.
The apps I disabled are: YouTube, TalkBack, Sound adjustment, Play Games, Picasa Uploader, Maps, in-store Demo, Hangouts, Google+, Google Text-to-speech Engine, Google Play Books, Google Contacts Sync, Gmail, Email, Contact Storage, Contacts, Chrome, Calendar and Calendar Storage. I just pressed "Disable, cleared storage and rebooted the device and I got longer battery life as well as it being more responsive to touch input.
If you need to edit your FLAC or mp3 files, there is a free program I found which is easy to use and works great, it is called Mp3tag and you can look it up. I use it to add album art (the higher res the better), edit artist name (such as Pat Metheny, Pat Metheny Group, Pat Metheny Quartet) where there are multiple names for an artist and just make them the same and remove fields I don't want or need such as album number when there is only one. I also use this program to change Genre to what I want and I try to simplify this so I combined "Rock, Pop, Rock & Roll, Pop Rock and you get the picture to just one Genre. In some cases, I made up ones that I like such as Jazz Piano, Solo Piano, Jazz Vocal, Electronic (Bonobo), to make things that I like easier to find. That said, the ZX2 will display each Genre and give you the number of albums in each and when you pick a genre it then shows you all the albums containing the specified Genre by artist and then by album cover and touching on the album cover brings up all the songs on that album of that genre. You can also choose your music by Artist name, Album name, Recently added, Release year, Playlists, All songs or Cover art view. So you can see that editing your files to suit your preference may be time consuming at first however, it makes navigating large music libraries much easier and enjoyable.
All in all, I am very happy with the NW-ZX2, it sounds great, can drive a variety of headphones, has good battery life and is really easy to use with it's big screen and touch interface.