Subsequent 37 CV - feedback not working?

Happy new year everyone.

I gotta say I’m not entirely impressed with this legendary Moog quality, one reason is the following;

So I bought a new Subsequent 37 CV and the Feedback knob doesn’t seem to do anything even when the mute/unmute button is lit and nothing is plugged into the external audio in.

The manual says to be careful because it may add a lot of volume. Under what condition should it make a definite audible difference so I can tell if it is faulty?

Edit: Well I tried to see if the audio in works - it doesn’t.
I plugged in a drum machine, turned on the audio in, turned up the knob, pressed some keys and nothing.

Unless I’m doing something wrong it’s pretty disappointing.