Hi there,
 
I would like to say few things about this little thing here. After seven eight months of use I found uDAC2 quite good when it comes to sound quality for this money (lets say money for pure DAC). Why? First I was listening the uDAC2 through AMB Mini3 (via line out) in a simple connection - connected directly to USB on my laptop - for a few months. Yes, gotta argee that after first impressions and the ear got used to it, the sound was nothing special. Not that boring or unbalanced, but neither entertaining. After some time, an idea occured, since uDAC2 has no external power supply possibility, you still can conect it to an USB hub with additional power supply (usually to attach external HDD) and hat would supply this little piece of DAC with enough power. And it really does. For example with 5V/2.5A power supply you can really hear a great difference in every sight. Details, dynamic/sharpness, bass response, soundstage together with separation in space, highs also.
 
Pitty is that there is hard to find some reasonable linear non-switching power supply unit with 5V output voltage to connect it directly between PC and uDAC2. I think I'll have to build one :-D
 
And also pitty that nobody here's considering that supply has major effect on sound quality in case of USB DACs.

Im not used in review writing, but since I am so happy with these cans, I have to write something
I bought used M50S (six months old) for very good price and in really nice condition. I've decided so because of lot of traveling around the world. And these are solid and nicely portable.
 
Together with Mini3 (wall connected) and uDAC2 fed through ASIO with FLAC files the sound is quite decent. Stage is there, instruments are not as detailed or revealing as on RE0 - but only at first sight. In fact, I found lot more details in soundstage, instruments or sound sources are separated, also there is really nice slight echo in the stage when it should be there. After modding (OXOOOXO mod on lower holes), mid bass is more polite, better behaved, not that disturbing like out of the box but still very low. But this is a question of taste. Overall sound is good balanced, higher mids and highs are good recognizable, enjoyable and it sounds like they have something to offer, even they are not that mouth-opening like in RE0 at first sight, but they are still clear (wait man! these cans are closed! ).
 
Definitely worth the money!
 
EDIT: After few months with these on a head, I found it better to add an extra supply for the uDAC2 (f.e. through USB hub) than applying the mod on cans. Results in details and frequency response are incomparable.

Pleased by the sound quality, almost perfect with Mini3. After three months I had to return it back, bad contact inside right driver I think. The cable is really tiny. Hope I'll see it soon in better condition.

Originaly built three years ago for my new RE0s to get rid of hiss and get some more bass from FUZE2 and I've hit the target accurately.  Anyway, after some amount of time, I found it to be easier and more reasonable to just simply set up the EQ in Rockbox. Now using Comply T-400 it's quite enough.
 
By now using in combo with uDAC2 and ATH-M50S when playing music from foobar (using kernel stream, resampled) or watching movies (direct sound). That means, it's always supplied from 15V linear PSU, so battery works as a filter, together offering a bit lower impedance at the output aswell. That's actually only good for ATs. Bass is very extended, but not excessive or annoying. Rather well controlled? Cannot really talk about highs, here seems to be influent the supplying method of the DAC (usb hub with pulse PSU), but sharpness and plenty of details that are about to be revealed are present.