Mackie 1604 #3 sub out not working?

[dragonworks]

My mackies #3 sub out seems to be not working. I have tried everything, I have had the board for two years and I am pretty sure everything is set correctly. If I remove the plug from the sub out and place it in the #3 direct out no problems.
Any suggestions before I tear the board apart and take a look?
I am using the elco connector.

 

[Recording Engineer]

Well, once you've adequately determined more than likely, it's NOT simply user error by thoroughly checking things over numberous times, it's time to open her up!

Hopefully, it'll be something rather obvious.

 

[dragonworks]

I have used the board quite a bit and doubt if it is US(user stupidity) and will probably have to open her up and do surgery.