Audio issues with my Behringer Q1002USB mixer!!!

I need help, people!!
So, I just finished repainting my room and doing all that nonsense. I rearranged the layout, set up my home studio, and now I'm stuck with a massive headache. My setup uses a Behringer Q1002USB mixer. Everything is wired correctly—my Adam Audio F5 monitors are connected via RCA cables from the L/R outputs directly to my audio interface. Here’s the kicker: when I try to use the main volume, the sound is terrible. It's this muffled, muddy mess with zero high end—just heavy bass and mids. But the second I plug in the USB cable, everything works perfectly fine.
There is absolutely no point in using the shitty integrated audio on my motherboard when I spent good money on a dedicated interface.

Everything was working flawlessly before I started painting and moving furniture around.
Is this some stupid software setting on the interface, or am I looking at a hardware failure?

Wait—are you actually using that mixer as your audio interface, or do you have a dedicated sound card hooked up separately? It’s a bit vague in your post. If you do have a separate one, try this: plug an external USB audio interface into your computer first and run the audio through that just to see if the issues persist. Once you've done that, run the same test using that USB Yamaha unit. Whichever device starts acting up would be my prime suspect. I’d also suggest swapping out the USB cables and trying different ports—keep an eye on those USB2 vs. USB3 connections and any potential compatibility hiccups there. Also, maybe double-check all the tone controls (EQ, etc.) on the mixer itself, along with whatever software you're running—including any specific driver software for the card (like PatchMix or whatever might be included)

There, hope that helps.

From what I gathered, he’s got the onboard audio on the motherboard plus that dedicated interface he picked up later. So, when he hooks everything up via USB, it pulls the audio from the integrated chip and everything works fine. But then, when he does this one weird thing—I didn't quite catch how he wired his monitors—the sound skips right to the external interface. If that's the case, where's the mixer in all this?