I've really taken a liking to the Resonessence Invicta.
DACs on this level (ie. "more than most sane people would ever consider spending") tend to be more similar than dissimilar to my ears. With a few exceptions they all tend to sound pretty good, and differences are more a matter of one flavor versus another. A bigger issue at this price point lies in feature sets, I think. Some USB implementations for instance are much better on some DACs than others, so if one is going to primarily run the DAC via USB, I'd definitely suggest some models over others (or recommend something like the Off-Ramp 5).
The Invicta is a flavor I really like. It's incredibly resolving, but it's also slightly relaxed. By that I don't mean it's dark or veiled or anything like that, but rather that it doesn't scream in your face. "Effortless" is how I like to describe sound like this. It's natural but still retains enough of that energetic snap that makes for an involving experience. It has a bunch of filters that work really nicely, some of which give a more relaxed presentation while others dial up on the resolution and crispness, though most of them still sound "correct" to me. This is where the Invicta takes the lead for me compared to some of the other DACs I've tried: that sense of naturalness and effortlessness. Having briefly auditioned the NAD M51, I'd say that it's a close second pick for me (and would be an easier recommendation at half the cost of the Invicta), but it sounds a little tight-arsed in comparison. Presentation isn't cavernous on the Invicta --- it's actually spacier on the Onkyo DAC-1000 --- but imaging is great, with a very clear placement of constituents on the soundfield. Compared to the Onkyo instruments have a better sense of weight, with a better delineated contrast to the sense of air between them.
USB 2.0 sounds good on this DAC. I've heard from some folks that it tends to work better with the Mac due to the drivers, so that's something to keep in mind (I'm using a MacBook Pro myself). I'd love to try it out with the Off-Ramp 5 to see how much improvement I could glean. The Invicta's UI is pretty intuitive and easy to pick up, and accessing the various filters and settings is straight forward. It's a little annoying that the amp can seemingly only be put into stand-by mode using the remote however. Aside from that, I like the way using this DAC feels. The SD card slot is also a really neat feature, one I'd probably take advantage of. In fact the DAC can be used as a stand alone unit without any other source or amp since it has a headphone amp built into it and can just read high-res files from SD cards. Interesting, the sound quality from SD slot is even slightly better than USB. I could keep files on SD cards to use with the Invicta, and then when I go out, I could simply take the SD card and use it with a DAP like the AK100 for a seamless transition.
The headphone amp built into the Invicta is actually pretty good, too. Usually all-in-ones mean compromise. This amp sounds better than a lot of stand alone amps, however. When you consider all the features packed into the Invicta the $4k pricetag is a bit easier to swallow. Or at least chew on. Om nom nom.
Of the four DACs I'm primarily considering, it's actually the least expensive. The three competitors in this case are the Berkley Alpha Series 2, the Meitner Audio MA-1, and the Resolution Cantata.