Hi, I have a receiver Yamaha HTR 6030 connected with optical TOSlink cable to external Sound Blaster Creative X-FI HD (to my PC)
My question is how do they interact with each other when it comes to sound quality? How does the receiver and how does the sound blaster affect the sound? Will I get better sound with better sound card?
Using the optical output of the soundcard puts almost all the responsibility for quality on the Yamaha. You are not using the digital to analog converters in the soundcard when you use digital audio out. The receiver is doing all the surround sound processing too.
If you had optical on the motherboard then you wouldn't need the soundcard. For gaming there might be some effects in the soundcard that you might want but that's about it.
USB DACs are used for high end audio applications so you don't need to use a soundcard and can send higher resolution files than spdif digital can usually handle.