OCC has published a new review of the Asus Xonar Essence STX

The noise level is very low for all soundcards, but the lead is taken by the Xonar cards, the Essence STX beating the rest of the pack. This trend continues until we get to Stereo Crosstalk, where the modded X-Fi XtremeMusic has better stereo separation from about 2 kHz upwards. In the 24-bit, 96 kHz tests, the modded X-Fi XtremeMusic has a strange rolloff at the audible frequency extremes, whereas the Xonar Essence STX has a much truer, flatter response. Again, the Stereo Crosstalk is the only test where the modded X-Fi beats the Essence STX, and it's only at frequencies above ~2 kHz.

Asus Xonar Essence STX Review