Right now I use AMD 4850 sound via HDMI on my receiver.
There is any reason to swith to my Xonar STX via optical cable? Or the quality will be same (receiver take care of audio)?
AMD 4850 graphics card?
So you currently are using an HDMI cable to connect the computer's AMD 4850 graphics card to the Sony receiver?
And you plug you headphones into the Sony receiver?
But your asking if it is better to connect the Essence STX to the Sony receiver, using a S/PDIF optical cable?
Why not just plug your headphones directly into the Essence STX, I'm sure it's better then plugging the headphones into the Sony receiver.
Or is the reason your asking because your looking to improve the speaker audio quality?
The speakers hooked up to the Sony receiver?
Then using HDMI is the better choice, over optical, for speaker audio quality.
When you send a 6-channel (5.1) or 8-channel (7.1) digital audio signal out of your computer, using S/PDIF (optical & coaxial) or HDMI, your computer is not really doing any "processing" of the signal, it's just passing it along. The Realtek & STX are not doing any "audio processing" to the audio signal. And HDMI is just a digital signal (audio & video) transport.
Optical is limited to 6-channels of 24-bit/48Khz of compressed digital audio, which should be good enough for DVD movie of gaming audio and Blu-ray movies should sound good.
HDMI can pass 8-channels of 24-bit/192Khz PCM (uncompressed) digital audio.
I'm guessing HDMI would make a difference for the latest Blu-ray movies.