It's almost certainly something at your end, so don't rush to send the unit in. Maybe you are connecting unbalanced feeds using a balanced-type cable. All three XLR or jack pins must be connected to something, otherwise you get (to quote the UM) more noise than signal. Same applies to the outputs.

If you are using 1/4" unbalanced, be sure to use a mono cable, meaning tip/sleeve, not tip/ring/sleeve.