The 6-pin cable provides clean power to the sound card, and minimizes possible interference. Please install the card as intended to avoid any possible issues. You could consider using a custom power cable for better aesthetics.
Since you have owned this card for a while now, I assume everything was working fine up until you disconnected the 6-pin power cable?
Tried the usual things such as reinstalling the card and drivers, or changing PCIe slots?
_____________________________________________________________
FPS, Racing, and VR Gamer / Tech Enthusiast / ROG Admin