I actually got to play Santa at work and get these for a coworker because of good performance on his part. He was cool enough to let me listen for a while and I really liked the sound. I've never heard an open-back design before and I really liked it.
 
So I listened to some Heather Nova and heard a loop in Ruby Red that I had never noticed before. It was pretty obvious. I played other stuff too and all the music had more distinct layers than I'm used to. I've got some good AKGs and I've heard the layering that the NuForce Icon HD brought to those cans. I was impressed to see that I got pretty close to that feel with these Grados unamped. Quite surprised and pleased. The bass was also quite nice. Only a few high treble sounds were a bit sharp. Maybe some burn-in will help.
 
I think these are really neat but I'm still mixed on the sound leakage issue with open back cans.

I didn't know things could be this good. Considering the criticism of NuForce, particularly around their uDac2, I doubted if they could make something good. I plugged my AKG's into this unit and the sound was wide and brilliantly detailed. The bass was alive and fantastic. The clarity was amazing. Nothing drowned out anything else (i.e. no bass wiping its snot all over the lower mids). I couldn't believe what I was hearing.
 
I played all kinds of stuff through this. I think I actually like piano music now because of this thing. I can feal the pianist play the music, instead of just hearing the resonant timbre like I usually perceive it. Also, did you catch how the violins in U2's Bad might actually be violas and are the real deal instead of some loop like I thought they were? How 'bout the fact that Daft Punk's Recognizer from Tron Legacy actually has some studio recorded "bead shakers" and it's not 100% electronic? Everything sounds new again. Oh, and with poorly encoded music, garbage in = garbage out.
 
I can't tell you what DAC or OpAmp setup they have, but it's impressive synergy. It's the kind of thing that folks like Benchmark charge 3x for... and put it in a bigger box too. I don't talk all audiophile snobbish with subdued language. I prefer to put it out there like this: I really like it a lot.

Keep in mind that my star rating is weighted against price, and I'm reviewing the uDac2-HP. It sounds just like my laptop's out even though there are legitimate reasons to expect it not to. There's a sense of quality to the unit and its volume knob from their feel but the USB cable that came with it is poking wires out of the plastic shroud. Lame-o. I have another cable hanging around that I used instead.
 
Anyway, I like the sound well enough despite its supposedly massive shortcomings that others talk about. It isn't spacious like the Icon HD (which is an awesome product) but it sure sounds fine with some of the hidden details in the recordings no longer getting drowned out in the poor analog circuitry of my laptop. It's also so nice to deal with a volume knob again even if I am limited to the top 40% of the range for good-enough balance.

These things hurt my ear canal too easily unless I use the orange foam inserts, but they get gross and deform a little too easily.
 
Anyway, the sound on these was good enough for me to realize that my iPod had a pretty constant hiss. They also made it apparant that I could only listen with EQ off or the signal would clip. I never listen to these things with the volume up. I'm a quiet listener so I don't lose my hearing, and the iPod's shortcoming became very apparant. So I rated them quite good at first and told people how much I liked them... until I realized that there was no width to the sound. Clarity was missing all over the place in the spectrum and bass is WEAK. The isolation was only so-so, and I've since heard much better sound from products in the $50-$150 range. I use SRS HD Audio Lab on my laptop to try to compensate for its shortcomings, and so far I like it. Using HDAL helped me find out that the bass response, as anaemic as it is, goes down to about 50Hz, so that was cool.
 
As critical as I've been, I've had them for 4 years and I'm not planning to get rid of them.

They're not hard to drive, they isolate the environment well, and they are the best phones I've heard so far.
 
They are lean on bass until you plug them into a good amp. I tried them with a NuForce Icon HD and they sounded so unbelievably detailed and pleasant with strong bass, clarity through the entire range, spacious sound stage, and clear yet unhard trble. I was blown away at how good these things really are.
 
I usually just monitor voice-over straight from the monitor port and these are fantastic in that arrangement because without an amp, they are nearly perfect in the human vocal range already.
 
They have two earpad types in the box and two interchangable cables in the box. I switched the pads for the velour because they're more comfortable (less heat build-up) and I like the way they sound a little better. It's subtle, but the difference is real.
 
I don't understand why they are as inexpensive as they are. They used to cost more... much more. I think they're worth more but I'm glad I didn't pay more. Overall, these are tough and great souding cans.
 
UPDATE: I posted three blog article about these in great detail at http://akgk171.wordpress.com/