You should be able to use your optical cable (Toslink to Mini-Toslink) for higher quality using a digital signal, using multiple 3.5mm-RCA cables would be analog and not an elegant solution.
Gigabyte GA Z97X Gaming 7 also has SPDIF output, so check to see if the output is going through there instead of the Xonar DX (you'll need a Toslink to Toslink cable). At the same time you can also check to see if the input on the STR-DE675 is set to the correct audio input (by pressing 'Input Mode' or 'A.F.D', should be analog by default so you need to change that).
Just make sure the correct playback device is select as default (Xonar DX), and input for the amp is correct as well (the Multi Channel Decoding indicator should light up for appropriate content)
Were you able to set up your surround sound system properly before? You've had it for a while
🙂
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FPS, Racing, and VR Gamer / Tech Enthusiast / ROG Admin