Went from GT 520 to GT 545, audio quality improved tremendously. why?

I have been tweaking my system by changing video cards I first tried a nvidia GT 430, then a GT 520 and was not able to hear any difference in audio quality. The next thing I tried was turning on the headphone surround effect in my VCL media player and got a very good increase in quality of sound through my old hi-fi which includes a powerful power amplifier and old Bose 901 speakers. I next tried nvidia GT 545 video card and the sound Improve phenomenally . The video card in particular I tried was the PNY brand using nvidia GeForce GT 545 chipset and 1536 MB DDR memory.

So what was the cause of this tremendous improvement in sound quality? Was it using the brand PNY for a video card? Was it GeForce GT 545 chipset ? Or the amount of hardware acceleration that video card has compared to the others I tried? (I had video acceleration overlay turned on in the VLC media player.) What happens is the dynamic range with my new jack up seems tremendously better. Human voices sound much more realistic. You can also hear a tremendous difference when listening to piano's every note sounding very vibrant. So please give it your best shot and tell me why you think this is happening. I plan on building a new computer shortly and sound quality will be a major concern.