I've had the AK Jr now for about 10 days and would like to add my experiences both pro and con. To cut to the chase it is a keeper though not without flaws. To begin with I have used it with Westone W40, Phonak PFE232, and ER4S IEMs and Sennheiser HD700s. My principal comparison was between the DX90 w/ Lurker0 and iPhone 6 w/HA-2.
To begin with, it is extremely well made and appears to be quite durable though I have not drop-tested it intentionally or unintentionally.:wink_face: I love the form factor as it is very pocketable, much more so than my DX90 or the IPhone combo. I also enjoy the UI as compared to the DX90. While Lurker's firmware make the screen readable, it is really too small vertically for touch use. I like that I can easily access the controls from my shirt pocket without removing it. The DX90 is too big and heavy to carry that way for me. I have upgraded to fw1.03 and with 5000 songs on a combination of onboard and 128G card find the speed faster than my DX90 but slower than the iPhone. I like the righthand fast nav feature that I wish the DX90 had.
One tip I discovered if running on OS X is to ensure the internal and card drives are excluded from Spotlight as it will double the number of files on your device if connected as storage by adding a hidden 4K file of the format "._". Besides taking up space they can impact scan performance. If this has occurred open a terminal and navigate to your music dir. Then enter "rm -rf */*._*" and "rm -rf */*/._*" without quotes assuming you are using artist/album storage convention.
What I do not like regarding the firmware is its sensitivity to music files. As previously reported, it can fail to complete and scan or loop if it finds a file it doesn't like and won't let you know which one. I have also discovered to my dismay that it will not display files with embedded png formatted covers
. In this case MP3Tag is your friend and has worked in all cases for me using JPGs even though I had to launch Windows. TidyMyMusic and Yate on Mac were not nearly as reliable for reasons I have not yet determined. Of course the DX90 is also picky about displaying art but is more tolerant when it finds a file it can't display.
To round out the negatives, I have yet to successfully get it to make a sound on my mid-2013 Macbook Air as a USB DAC even though it is recognized as such. I have tried various cables down to 4" and still no go. Would like to hear if anyone has been successful. Works fine on my Mac Mini. Both are running Yosemite.
On the plus side, while some have reported lockups, it has been rock solid for me thus far. I like the switchable line out and digital volume though I would like the ability to have a selectable taper as it does not appear to implement a loudness contour and thus can be arduous to make large changes. Battery life has be fine for my usage pattern as it has never failed to last throughout an entire day.
Onto the sound, I like a clean neutral sound with extended but not emphasized extremes. I have enjoyed the DX90 especially with the Phonaks. However, found it didn't have the toe-tapping flow with my WD40s which are significantly more efficient. I do believe the DX90 drives my HD700s with more control that the AK but both can generate sufficient volume for my tastes. Where the AK excels for me is in combination with the WD40s on well recorded/minimally processed music as it becomes an immersive experience. I literally feel the music below my head and around me. Listening to 24/96 AIX recordings that I have has sound simply "right" with excellent low level resolution. Excellent examples include albums by Atrium Musicae de Madrid, Laurence Juber, Stephane Wrembel, Jesse Cook, Sharon Isbin, Ejii Que & Miinn Orch, and Katie Melua to name a few.
As far as scaling up, I find with my HD700's it does sound better when running through the HA-2 which as a hell of an amp for its size. Its line out is excellent. While I briefly considered foregoing the DAP for the iPhone/HA-2 combo, I find that for portable use I don't like giving over my phone entirely to music and like the flexibility of the combinations. Now if I could get the MBA to work with the AK (the HA-2 does) that might change.
Hope everyone finds this useful.