Sounds like it could be a cabling problem…. You should be running the output balanced; If my memory is right there’s jack outputs, and you need to either use standard unbalanced jacks or balanced (3-pole) jacks. If the ‘ring’ is left disconnected, then the output may be low and thin.
If it’s not that, then it sounds like a real fault….. I fix all faults on gear of my design at a fixed rate…. #25 plus the cost of sending it back to you. I’m happy to look at it.