Metralla's first sentence says it all. What cables were you using? Were they the same model? What gear was being used?
Even though a product has XLR connections, it may not at all use a balanced circuit topology. For cases where a single-ended signal goes through several additional stages on the input to output just for the sake of driving XLR cables is just plain senseless. Unless there are circuits for each phase, I'd stay with single-end connections.