The Oxygen driver supports the Xonar Essence ST. How about the STX, which is the PCI-E version of the Essence ST ?
I guess it's probably the same, but some PCI IDs can prevent the driver from working with the STX while working with the ST (although that the ST is slightly different from STX, but not on the processor side).
If anyone knows...
EDIT : sorry, found the Essence STX listed in the alsa "supported cards" matrix. This means that the STX is supported.