You can check everything with the probes of the DD-1 plugged into the amp outputs. Zero reason to check everything at each point.
Set the amp and LOC gain all the way down
Then check max clean output of the headunit at the amp with the 0db track (which very well could be max).
Then with the headunit at that volume setting and using the 0db track turn the LOC gain up until you get distortion.
Then do the amp using a -5 or -10db track.
Done. Super simple, don't make it more difficult than you need too.