In the world of portables, Astell&Kern is a bit like Apple: you're paying a premium for more than just performance. However, I've found that the price trade-off is very much worth it.
Fit and Form
At $300-$400, it ain't cheap. Given that price, Astell&Kern have not left a single part of this device feeling cheap.. It's design is mainly aluminum and glass, and is very elegant. I usually get something around 10 hours of use out of it, which is partially dependent upon which headphones I use and how much power they require.
The device is very easily portable, and its front is dominated by a large aluminum chin and it's 3.1" touchscreen. The tactile volume on the side is a godsend; it is tremendously satisfying to tangibly click from one level to the next (of which there are 150).
Some have complained about the sharp corners of the device, and before I got a case for it, I agreed. They sell a leather case that alleviated this issue immediately for me.
I have had some issues with the relatively sluggish OS on the device, but nothing that has impacted its sound, and nothing that goes beyond a little bit of lag.
This device shines in its specifications. It natively supports up to 24-bit/192kHz files as a standalone device and 24-bit/96kHz when used in its USB DAC mode. It supports a massive array of filetypes (WAV, FLAC, WMA, MP3, OGG, APE, AAC, ALAC, AIFF, DFF, DSF and AIFF) and while it won't natively support DSD, it will convert them to PCM.
All of these files are ran through a Wolfson WM8740 DAC.
On to the sound itself, I've found that its dynamics and separation are very welcome. Instruments spread out slightly and become noticeably more distinct when compared to a phone used as a portable player. Bass is incredibly well timed, and it spreads out and opens up the midrange very pleasantly. Treble is precise and brisk, with no noticeable sibilance to speak of in the majority of the headphones that I've used with it.
Key descriptors: clean, detailed, engaging, textured, and open
Minutia and Miscellaneous
The AK Junior's 1.95V RMS got plenty loud enough on every headphone I threw at it. Being able to simply drag and drop tagged files of any filetype onto the device and have them load properly was a wonderful convenience. I have kept my onboard library lower than the 64GB of storage on the device, but know that I can add up to a 64GB SD card if it ever becomes necessary.
It also supports Bluetooth 4.0, but I've always used my phone as a source for car audio, so I only used that occasionally.
This device provides wonderful sound in a beautiful package. It is expensive, and that's definitely a factor in everyones' minds, but it delivers on all necessary points, and does so in spades.
This device was not provided to me by Astell&Kern, it was purchased purely of my own accord. I am not being paid by anyone to write this review, to endorse the product reviews, or for the content that I put in the review.
Headphones used with this device include: Xiaomi Piston 3s, Audeze iSine 20s, Sennheiser IE80s, Audiotechnica M-50x, Hifiman HE-400i, Audeze EL-8 Open, and Beyerdynamic DT880 250 Ohms. I have had this device for the past two years, and it has been my main portable device for that time.