How did this NOT result in a suicidal feedback loop? Lucky or not?

My Steinberg UR22 interface’s gain knobs have no dB marks anywhere. So, I decided to play a digital white noise test signal at 0dBFS from my DAW through the UR22’s stereo line output-ports, and loop the cabling around to its stereo line input-ports. By analyzing the waveform coming through the line inputs in my DAW, I was able to determine where 0dB is on the stereo line output-gain knob, and stereo line input-gain knobs.

This is great, but then I realized that my interface has a “mix” knob. According to the manual and its block diagram at the end, the “mix” knob can be turned fully left in order for any sound entering the stereo line-input ports to bypass the DAW and head straight out the stereo line output-ports. Presumably, this is so a musician can compensate for the lag caused by a high audio-buffer setting in his ASIO driver. Regardless, I’ve noticed that even if you turn the “mix” knob fully right, a very small amount of sound still bypasses the DAW and heads for the stereo line output-ports. It makes sense; it’s a “mix” knob, not a switch.

It begs the question though… if sound from my DAW is heading out the stereo line output-ports, only to be looped into the stereo line input-ports, wouldn’t some of the sound coming back through the stereo line input-ports be sent out the stereo-line outputs as well? If so, wouldn’t this create a feedback loop that constantly grows in volume?

Shouldn’t my interface be dead? Instead, it all worked fine. :confused: