Beware "Audio Enhancements" on Windows Inputs
TLDR - If your audio interface or USB mic sounds weird or bad, go to System -> Sound -> Input, and selct your device. Scroll down and shut off audio enhancements.
Out of nowhere my microphone started sounding bad. I have a mic preamp and processor that it goes through before my interface. It was muddy and was completely cutting "ess" sounds. It sounded like hot garbage. I thought my processor was the problem. So I messed with it until I got something close to good, but still bad.
My interface is a Mixcast 4. I plugged the mic directly into it with no processing and just enough gain to record it in Audition. Still hot garbage. But still the same complete deletion of ess sounds, particularly when I hissed into the mic.
That's when it dawned on me. There must be something in windows screwing it up. That's when I discovered "Audio Enhancements." Apparently these are designed to alleviate bad audio from webcam mics and suppress background noise. But it also explains why some people, who I send my USB mic to for guesting on our show, will sound terrible while others sound great.