Looking at the specs for my amp and in some general reading, I see that the input/output impedance, voltage, and sensitivity are all about double for the XLR connections. I am not sure if those stats equate to a sonic difference. Here it is for my pass labs amp:
Input Impedance: 50 / 100 (SE & BAL kOhms)
Input Impedance: 50 / 100 (SE & BAL kOhms)