Remove the Xonar under device manager (uninstall device), then Action > scan for hardware changes.
Sound cards don't just partially give up like that; JBI is right.
Just curious - do you need the soundcard for something specific, like analogue capture, high-impedence headphones or something else unusual? I only ask because the largest improvement to sound quality on PCs and laptops is to get the DAC portion of the sound system away from the rest of the electronics. Modern motherboards are better than they used to be but no matter how you slice it, there's a lot of EMI inside a PC case.
Uninstalling and reinstalling worked. Thank you. I feel sheepish for not thinking of doing this myself. It's been so long since I've worked on this stuff I've started to forget some basics.
Part of the reason I got the STX is that it has an EMI shield. And I did have some unusual audio demands that had me wanting to offload audio to a dedicated processor. At one point I was doing some acrobatics with Virtual Audio Cable and capturing gameplay both on consoles and PC and I'm remembering the STX offered advantages to some of the problems I had to overcome. Mostly I got it because it was well-reviewed and I had poor experiences with motherboard audio I wanted to avoid when building my current system. Namely I needed a SPDIF output that could handle Dolby Digital Live while also offering an internal virtual surround for some of my favorite PC titles and quality playback for music via headphones.