Elite Bastards posted a review on the ASUS Xonar Essence STX PCI Express sound card

From left to right, we find that the Essence STX carries analogue outputs for left and right speakers via RCA outputs (with an RCA to 3.5mm jack adapter provided with the board) - If you're using analogue speakers, that means you'll only be able to produce two-channel audio from the Essence STX. Moving on, we then find the card's headphone output and line-in jacks, both of which use 6.3mm jacks (with one gold-plated 6.3mm-3.5mm adapter provided as part of the STX's bundle), and an SPDIF output with support for both coaxial and TOSlink connectivity, and allowing for multi-channel audio via Dolby Digital encoding.

ASUS Xonar Essence STX PCI Express sound card review