Ok I have never used the 1680 before but I'll try.

Usually you want to send your signal through the post fader aux send. Go to the specific channel(s) aux sends and

1. Turn up the aux send level
2. switch it to post fader

Then there must be a master aux level in the master section. Turn the master aux level up according to which aux you are using. Usually the last 2. So if you are using Aux3 turn up Aux 3 master. Try that and see if it helps.

There must be a way to save the editied fx patch on the 1680. You should consult the manual.