Slater
Headphoneus Supremus
It's a bit too early to say much, but the thing that immediately jumped out at me is that when I boot into Rockbox the sound is warmer than in the stock firmware. I checked all the Rockbox settings, but everything is set at default. I have no explanation for this. I prefer the stock firmware, which makes no sense because, well, Rockbox! Rockbox rules! I'm normally rather sceptical of differences between amps, DACs, cables etc, but I hear what I hear and the difference is obvious to me. I'm fairly certain I could distinguish between the firmwares if someone did a blind test on me. Weird.
Another thing that kinda bugs me is that it's actually pretty hard to get the microSD's into the unit. They "click" in place a bit too deep inside the housing, and the only way I can get a card to stick is by using a tool to press down on the microSD when it's in the slot.
SQ is good though. Soundstage is noticeably wider and more immersive than on, say, a Sansa Clip+. I'm happy with the Xduoo X3 and for the price I paid ($55) I feel like I got a nice and noticeable upgrade over my current daps.
There are some sound tweaks from the default you should do with Rockbox on the X3. I'll post back in a little bit with the settings.