Hey Razamvmt,

The MIDI button in the Serato DJ interface is used to MIDI map secondary controllers. When you connect you primary controller, in this case the NS7, it will be set to a default MIDI map as determined by the manufacturer. If you connect another device the MIDI button will light up and you should be able to remap the secondary controller.

Also the channel gains in the Serato DJ interface are a representation of the tracks levels before it gets sent to the controller and are not mapped to the gain knobs on the NS7. The gains are there so you can adjust tracks that are too loud or quiet, the gains you set are saved in Serato DJ so next time you play them the levels will be more even.

I hope this helps,

Cheers, Sam