This and that:
The screen protector and leather case are unfortunately optional. The latter is definitely of high quality.
Build quality of the CNC-milled aluminium chassis is good. The back is made of plastic though, so the player feels rather light (although not as light as my Apple iPod Nano 7G). It's rather small and feels good in my hands.
There are no other outputs except for the 3.5 mm headphone jack. I don't mind and don't need any additional outputs anyway.
The little LED next to the on/of/standby button is a nice addition and can be disabled in the settings.
Dedicated volume and playback control buttons.
64 GB of built-in memory are nice and can be expanded with a micro SD card of up to 128 GB. The limit and that it only supports FAT32-formatted cards feels definitely outdated these days and for the price.
The touchscreen's resolution is a bit low (240 x 320 px) but still okay for a DAP, but don't expect IPS. Responsiveness is good though and the J feels decently quick (not as quick as a smartphone though) and definitely quicker than the Plenue D that I used to own.
In terms of user interface design, the J is just like the D, which is good, as it is intuitive and also included a search feature. The JetEffect EQ and DSP suite is implemented, too.
Gapless Playback works just as it should using FLAC files.
Shorter battery life than the Plenue D but still more than plenty with ~ 50 hours.
Volume Control:
100 digital steps with a precise scaling of 0.5 dB per step over the entire range. However, for me personally, the lowest possible volume is too loud with most in-ears. That's where pretty much all of Cowon's players, in all price ranges, already fail for me.
Sound:
Needless to say, the JetEffect settings were disabled in the following few paragraphs.
Frequency Response (unloaded):
Flat and neutral. No surprise here.
Output Impedance (Load: Ultimate Ears Triple.Fi 10):
The calculated output impedance based on the deviation is around 0.5 Ohms, so it's suitable for critical IEMs without a flat impedance response and backs up Cowon's own specs.
Hiss:
Using extremely sensitive in-ears, it is not as quiet as my iBasso DX90, the DX200 or Cowon's Plenue 2, but not hissy either. About similar to the Plenue D I owned, and among the quieter and better players when it comes to hiss performance.
Subjective Sound Perception:
No surprises. It sounds just as neutral/clean as any audio device without any flaws should.
Conclusion:
More or less an evolution of the Plenue D but clearly more responsive when it comes to screen inputs. Good hiss performance and low output impedance. Feels a bit outdated overall, though. Solid choice for purists. I clearly wouldn't want trade it for my iBasso DX90 or Apple iPhone 4 though.