This problem wasn't as much of an issue when I was streaming Animal Crossing, as that's mostly just ambience, but in a game like Detroit it's especially disruptive.
I did some research and found this: "turn monitoring off on your sources in Advanced Audio Properties". According to this though, audio monitoring is already off:
This was taken post-stream and so I'm not sure that, because it's no longer in use/plugged into anything, it reverts back to this by default?
Muting the audio via my capture window mutes everything - could it be that the echo is a combination of this window and the OBS visualiser being open?
If it helps, this is another look at the audio settings (but again, I'm unsure if this changes when I fire it all up):
I only post this here because I've no trusted source of folks outside of this forum.