Now I just have to figure out whether I want to get another external soundcard for my gaming rig or if I can live with switching the USB cable back and forth between the two PCs or if I can get used to control the volume with software. I really like hardware knobs though

I don’t know what you’re doing, but if your headphones are using a 1/4 inch jack or 1/8 inch jack and your devices have some form of audio output already, you can just use a plain ol’ analog mixer. That’ll give you knobs and let you hook multiple devices up.

I keep my headphones plugged into one. I rarely actually use the other devices, but sometimes I’ll want to be listening to a radio scanner or a shortwave radio or some other things, and it lets me just pipe whatever into the headphones.

searches Amazon

www.amazon.com/…/B0DDKBJFWY

That’s $11 and looks like it’ll do 4 1/4 inch inputs. Looks like they also have RCA and 1/8 inch input versions (though those cost more, and there there might be more-inexpensive mixers).