Here is a review that was just posted on diyaudio:http://www.diyaudio.com/forums/vendor-s-bazaar/321303-applepi-dac-performance-raspberry-pi-dac-post5520363.html
I was part of the ApplePi tour and want to share my impressions of this DAC.
I tested the hat with Rpi 3B, IanCanada IsolatorPi, and Allo Kali. Each board had separate power: I fed the Pi with an iFi iPower and the other three boards each got a GStew modified Jameco linear wall wart. Listening environment was a rather analytical nearfield desktop setup--a Tripath amp into some Axiom 2 way bookshelf speakers, and a humble full-range setup using another Tripath amp with Aura NS3 drivers.
First surprise: This DAC seems to be immune to PSU noise! Other DACs I have tested are all very sensitive and exhibit hum and hiss to some extent, even with an isolator board, Kali, and four separate power supplies with earthed ground rails via John Swenson Ground Tweak. Using these careful measures, I have been able to reduce noise by, say, 95% with the other DACs. But I can still hear a tiny bit near the speaker when music is stopped and I crank the volume knob to max.
In contrast, ApplePi is silent. Even if I float the PSU grounds by unplugging my earth wires from the negative terminals...it stays silent! The other DACs would be intolerably noisy this way. I'm impressed. This could make getting good sound from a Pi a heck of a lot less hassle and expense.
(edit: Since this test I have booted the iPower out of my setup completely, and going completely linear in combo with the IsolatorPi killed noise at the speakers almost 100% for any DAC Hat on my Pi stack )
I played music through the ApplePi with both Moode and Volumio. I was not able to get 16/44.1 files to play natively on ApplePi with Volumio, though Leo told me this works for him. It did play 24-bit files quite nicely, but everything in 16-bit I had to upsample...and IME Volumio upsampling sounds sort of lifeless. Thus, I spent most of the time playing my music collection (which is 95% Redbook FLAC) via Moode, which worked flawlessly, natively decoding all bit depths and sample rates I tried.
One huge advantage this DAC holds over all others I have tried is the beastly 2.5V output--it just makes both of my amp/speaker combos sing, almost like having a preamp. This results in some of the best, most effortless bass control I have heard in my systems to date. I wish every source component had such gusto. In both my systems, ApplePi made my little flea-powered Tripath amps able to push significantly more air from the woofers without distortion. (Note to self: buy a real amp). IME good gain staging just makes everything downstream happier. Take note: the board gets quite hot. Leo said he has made some revisions to help dissipate heat better with the production boards.
Entering into highly subjective territory, now. My listening impressions parallel my experience with Alex's DIAL_DAC to a great extent...the sound signature shared between the sibling Burr Brown chips is evident. Analytical and smooth at the same time--detailed without harshness. Especially with 24/192 files, transparency is excellent.
I feel that the powerful, controlled bass and detailed treble are the most apparent and expressive parts of the music with the ApplePi. At this point I have to reveal myself as a total sucker for midrange body and warmth. Thus, the ApplePi doesn't push all my buttons...in fact, I'd say it stands in slight contrast to the meatier tonality of the TDA1387 DACs which usually sit atop my Pi stack. Which flavor you'll prefer will surely depend on the other gear in your system as well as your personal taste.
Nice job, Leo.