There's always the possibility of inductive coupling of hum from the power transformer in a nearby power amp or another component into the 950, or it's interconnects. The test for this would be to move the 950 a foot or more away from it's present position and see if the hum varies, moving the interconnects around too. It would make sense that switching to a matrix setting would increase the relative hum, since the signal is being sent to more of the speakers. Like I mentioned before, both in my installation, and looking at the noise floor on an ocilloscope, there is no hum present in the ouput of the 950s i've had.