Obviously, you may have a hardware problems, but it is more likely related to how you are using it. It's not really clear how you have it connected, but my guess is that you have one channel before the amp, and the other in an effects loop of the same amp.
Unless I am mistaken, it only makes sense to do this in PRE/POST mode – anything else will probably feed back.
My guess is that you are treating the unit as two unconnected mono systems – this is not what it is.
