Computer make and model: Custom build. Ryzen 3600X, B450, 32GB DDR4, 500GB NVMe
Operating System: Windows 10 Home 1909
Interface: Presonus AR12C, v1.04
Short version: Is it possible via firmware update to omit the USB Return from the digital Main L/R send? The goal is to make the computer's input a "mix minus" that reflects everything except the computer's own output, while not affecting any of the analog functions of the mixer.
Long version: I think it will be easier to explain the change I'm requesting if I first recap the way things currently work as a comparison point. I will be speaking in terms of using the AR12C with Windows, but I believe this applies to the entire AR series and that the operating system is irrelevant.
The Presonus AR12C is a 14-in, 4-out audio interface. This means that when you plug it into the computer, the OS sees seven stereo input options (called "recording devices" in Windows) and two stereo output options (called "playback devices" in Windows). The input options are channels 1-2, channels 3-4, [...], channels 11-12, and Main L/R.
Slight plot twist: If you look at the mixer physically, it has 14 channels. Channels 13-14 are on a "super" stereo channel that combines inputs from RCA, 3.5mm, SD card, Bluetooth, and (most importantly for what I'm after) USB return. When the USB return option is selected on the mixer, the fader for channel 13/14 serves to control the computer's audio output.
Since feedback is bad and looping the computer's output back to its input is dumb, channel 13-14 is understandably not available as a recording device in the OS' sound settings. However, if you want to be able to send a full mix through something like Discord or Skype (remote lessons, perhaps?), this requires selecting "Main L/R" as the OS default recording device, since Windows can only use a single stereo input at one time. However, this option ends up sending the computer's output back to its input because Main L/R includes the USB return on channel 13-14.