Okay, I've been playing with the Essence for a little over 2 hours now, and I must say I am impressed. It's definitely a step up from the HDAV I've been using, which is in turn a step up from the Xonar D2 and Prelude.
My current setup is as follows:
Hardware:
Sound card -> Blue Jeans LC-1 RCA's -> Meier Corda "Blue" HA-1 MKI, crossfeed in the "off" position -> Beyer DT990 250ohm, plugged into the 0 ohm output.
To make things fair I pulled the LT1361 opamp out of the HDAV that I had installed and put the stock LM4562 back in that it came with. I also haven't played with the STX's built-in amp yet. Apples to apples.
OS: Winxp SP2
Player: Media Monkey 3.0.4.1185, Standard Ed., Otachan ASIO plugin 0.67 SSE2
Installation was a snap. Install card, turn on PC, cancel auto-detected device, insert CD, install software, reboot. The STX plays nicely with the Xonar HDAV that is already installed, but that's not surprising since earlier this Fall I had a Prelude, Xonar D2, and Xonar HDAV all running simultaneously.
Now for the good stuff. The sound.
The overall tonal sound signature is very similar to the HDAV (and thus it will be similar to the D2 & D2X as well) with one major difference. The Mids. Mids are significantly more present in the STX than the HDAV, leading to a slightly fuller/darker presentation. It's not a huge difference but it is immediately noticeable and is not overdone IMO. It's a welcome improvement. This is not to say that other parts of the spectrum are laid back/veiled, quite the opposite actually. Mids are also more controlled on the STX.
Almost everything else is improved on the STX over the HDAV as well: The highs are clearer and have more detail and a more 3D feel to them, which in itself is impressive given the opamp used (LM4562), which I found to be somewhat flat and "digital" sounding when installed in the other cards I've heard it in (Xonar D2, HDAV, Prelude). 3D imaging on the STX is improved as well, background/accompanying intruments that might blend into a nondistinct backdrop of sound on the HDAV are pulled forward into the room on the STX. The soundstage is about the same size on both cards but instruments are more 3D and more focused on the STX, and sounds have a more natural/extended decay and allowed to breathe a little more than on the HDAV. There is also more air in the environment in general and more cohesiveness in the msucial presentation as a whole.
The bass is pretty even between the two cards, though. I don't think the STX goes any lower. It's also not much more prominent than the HDAV but there is slightly more detail and texture than the HDAV. Mid/uppser bass might be slightly more prominent too.
What's also impressed me is that it is this good right out of the box. Both the Xonar D2 and HDAV I had needed about 30-40 hours burn-in time before they reached their sonic potential. And the Prelude took over week IIRC. The STX sounds better than them all from the start. I can't wait to hear what it sounds like after using it for a week.
Another big surprise was how good it sounds with the stock LM4562. Don't get me wrong, it's a good opamp. But like I said I found the highs it produced to be "digital" and somewhat flat image-wise in the other cards I've heard it in. In the 3 other cards the highs were also quite scratchy/fuzzy for the first several hours of use. Not so with the STX. In fact, for the first couple of minutes I had the STX installed, the HDAV still had the LT1361 in it (which fixes the LM4562's "problems" in the 3 above-mentioned cards), and a few quick a/b's showed the STX/LM4562 matched the HDAV/LT1361 in every respect.
Okay, that's enough for now. I'm off to play some more.