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.