Wanted to bump this thread up and post some quick impressions of the ZxR I received last week.
To keep it simple: sounds gooooood.
I was replacing a FiiO E10, which served its purpose for nearly a year and which itself replaced a Xonar STX. I use Denon D5000s, my personal favorite can.
For a little background, I was loving the STX but decided to ditch it for a FiiO after so many people around Head-fi were saying that the STX doesn't play nice with low impedance headphones like the Denons. Plus, I wanted a physical volume control because the software dial was a pain to use in the middle of a game, movie, or when listening to music. Got the E10, was pleased with it overall yet noticed that my sound quality took a hit--compared to the STX it was muddy at some middle frequencies and there was less instrument separation. A lot of detail and warmth was lost as well. The most annoying part of the E10 was that half of my songs, which are all in FLAC and I deemed to be good quality when I was using the STX, began to sound dull, quiet, lifeless, and tinny. I would have said that the E10 was more analytical than the STX, but since I lost so much detail as well as warmth, I can only say that the E10 is plain inferior.
So finally, the ZxR comes out with its ACM and I think "bingo!" Glad I went for it.
First, just like the STX, the ZxR puts out excellent sound for a computer audio solution. Great detail, a somewhat warm signature, but full--really brought out the best in my D5000s. ALL my sound library sounds great again--there are no longer those songs that I have to skip just because they sound terrible and lifeless. I use Coldplay's Princess of China to test clarity and detail because it has many complex and competing melodies and sounds as well as a lead male and female voice. I was relieved that again each sound was its own and there was no more garbled mess in the middle like there was when I listened to the exact same file with the E10. I even feel like the previously-garbled mids opened up Rihanna's voice and made it more lively and clear.
The most interesting thing for me about this all, and that I hope saves others a bit of cash is the compatibility of the STX and ZxR (which has most of the same internals as the STX) with low impedance headphones like the D5000s. Though there is one catch, I'm happy to say that they work wonderfully together. Some have said that the (now old) Denon line, with a 25-ohm impedance, were a bad match with high minimum impedance sources; the STX (and presumably the ZxR) do 10-ohms. Luckily this does not seem to matter for more stable headphones such as the D5000s.
Cheaper headphones may exhibit encroaching lower mids or a muddier sound when paired with such a high impedance source but higher quality ones seem absolutely fine. And in my experience, the D5000s (and also the D2000s) sound much, much better on an STX or ZxR than they do from cheaper but better-paper-matched sources like the E10. Don't hesitate to get a ZxR if you're using better-quality low-impedance cans.
Rock on, ZxR!