Hello,
The only level controls on the H90 are for instrument or line level. You should have the H90 set to line level if connected directly to a mixing console.
Here is a setup guide that may be useful: https://cdn.eventideaudio.com/manuals/h90/1.9.7/content/setup/dual-insert-daw.html
You can connect the mono aux output to H90 input 1, and outputs 1-2 to the return channels, and that will give you a stereo signal.
You will need to individually adjust the aux send knob on the console for any track that you want to be sent to the H90.
