Charlie T, Have you found a solution for the Essence STX card? I know it has been a while. I just found this post because I'm trying to see if the Essence STX II can do what you are trying to pull off, that is, have the same audio go to both the analog and the S/PDIF at the same time. I don't own this card, so haven't been able to verify, but in the manual, I found this:
Problem: "I cannot hear any sound from my analog speakers."
Solution: "Check if you are using a Dolby Digital Live encoder through the S/PDIF output. If this is the case, the analog output is muted to get rid of any interference between your digital speaker system and analog speakers or headphones."
Soooo, based on this, using the Asus "Audio Center" program that came with the Essence STX, the "S/PDIF Out" on the Essence STX should be set to PCM and not Dolby Digital Live.
Maybe you tried this already... Hopefully it works?
Cheers!