Audio MIDI Setup: I cannot choose individual Inputs in Aggregate Devices nor in an external Soundcard.

I have an SSL 2+ Soundcard. It has 2 Inputs and 2 Loopback Inputs (the desktop audio, the audio that goes "out of the computer"). I want to be able to separate these inputs, maybe creating 2 Aggregate Devices choosing just ones or the others.... but it is not possible! It doesn't allow me to select/unselect anything.

Any idea about how to do that?

Basically I produce music, and want to do Live streaming about it. But I want to have 2 different sources (one with Inputs, other with Loopbacks) in OBS or Streamlabs so I can duck the signal of the music from Ableton with my voice.

I can't believe Apple doesn't allow simple changes and tools like that.