This is an edit to my previous review. Basically, I didn't realize that I had to reconfigure my audio settings to use a surround sound setting. For me, I used Dolby 7.1, which split out all of my 8 channels in the audio mixer. Once I did that, rematrix-filter worked flawlessly, and was able to mute all inputs except for the master channel.
To describe my setup:
I have a Traktor Kontrol Z2 mixer with 8 input channels.
I only wanted to capture the master input which is the mix of all the other inputs. Without the Dolby 7.1 audio setting, and without rematrix-filter, I could not do that. OBS would clamp on all the inputs, including the phono inputs. So what I did is filter out (muted) every input except for the master input. Brilliant!