I have the Essence ST. It was released after the STX and claims to reduce clock jitter more than the STX. The ST uses a PCI slot and the STX is PCIe. Most at Head-Fi that have heard both cards prefer the ST. I also have the daughtercard for the ST and use mine in a 7.1 configuration. I use J. River Media Center for bass management, distance settings, level settings, and parametric EQ so I output directly to my amplifiers.
I am using my card with Windows 74 x64. When using ASIO output the card has automatic bit-matching with the current beta drivers. I use the
Asus Xonar Universal drivers and like them better. They offer lower latency and fix some issues with ASIO.
I did not like the standard opamps (they were too harsh) and have changed mine out. I can't recall off the top of my head what is in there right now. The card is extremely detailed and has a great soundstage. I don't know how much you have to spend to get something better, but I've heard the Benchmark DAC (with the same music) and I prefer the Essence ST.
I agree with Amir about the computer you use. Stereophile used two different computers and different OS's for their "comparison" of the STX and ST. I don't think the ST was getting the best computer for its capabilities in the testing. I use a Gigabyte motherboard due to their PCB design and components. I also use a Corsair 850 Watt power supply due to its exceptionally low ripple on the 5 volt rail. The Essence ST and STX both use their own power input for the audio output section. This allows them to draw power directly from the power supply rather than having it go through the motherboard.