Sounds like either insufficient drive gain at your external amplifier, an incorrect headphone-amplifier gain setting, or a serious hardware issue with your STX'™ analog-output section. I have used an STX™ for over a year in an Asus® CM1630-06 and it has performed as advertised, once the drivers are installed (see also my caveat
Post #4778 concerning attempting to install the Maxed Tech® UNi XONAR® Audio Software when virtual-webcam softwares are present); I normally run the RCA outs ("2 Speakers") through a Pyle® PCA2 amplifier into a pair of Realistic®/Radio Shack® #40-2054 two-ways, with the internal Texas Instruments® TP6120A2 on the 6.3mm TRS jack ("Headphones") on standby for any of a number of headphones I use.
Failures of the dual op amps in the I-V section are rare but not unknown. The JRC® 2114D and its competition have similar weaknesses to most linear integrated circuits; same applies to the National Semiconductor®/Texas Instruments® LM4562N and competitive dual audio amps in line-level buffer position. As I understand things, the Asus® XONAR® Audio Center™ can set the headphone-amplifier gain for different load impedances; three gain levels are provided.
"Hard" hardware issues for the XONAR® Essence™ Series may involve the actual resistors and capacitors of the signal chain from the I-V section to the headphone jack and RCA line-level outputs. Functional headphone jack and dead RCA's indicate a failed line-level buffer dual audio amp, while a dead headphone jack and functional RCA's indicate a failed headphone-amplifier IC.