Question

Hi, I just got a GAP MKIII Plus pre and I'm connecting it to my 1818VSL through a 1/4" TRS cable to one of the line inputs. I was wondering what I should set my channel output to on the 1818VSL to get the clearest sound from the GAP preamp

Answer 1

All you should need is to connect your external preamp to available "line" inputs on the front of the AudioBox 1818 with a 1/4 inch audio cable.  You have already done so it seems.

Outputs on the AuidoBox shouldn't matter.  These need only be use for your speakers, headphones and DAW playback.

Tho you mention "clearest sound".  This is relative term.  Relative to all connected equipment.  The general rule of recording is to increase the gain as high as you can without "clipping" your gear or your DAW.