The only way I know is by using the Unviversal Control app. If you are using a Mac, hold down the Option key, then with your mouse, click the fader in the Unviversal Control app that you wish to set to 0db (unity gain). The fader will then jump to 0db (unity gain). I am not aware of any way to do it on the hardware itself. I've wanted to do a feature request for this for a while now. Nevertheless, I hope this helps.