IF your question is how to use your NAD integrated as a preamp only: there should be two links (rectangular, black plastic covered) between the C356's rear PRE OUT 2 and MAIN IN RCA jacks.
Remove those and simply plug a pair of interconnects, between the PRE OUT 2 jacks and your new amp's input jacks.
If your desire is to add a second amp: use interconnects from PRE OUT 1, to that amp and leave the PRE OUT 2/MAIN IN links alone.