That's some quick service!

Since the hum cropped up when the rears and sub arrived, there are two possibilities. One is that one or more of the surround speaker cables and/or subwoofer interconnect are getting too close to a power cable and picking up some noise from that. The other (and probably more likely) is a ground loop due to a grounding issue between where the sub is plugged in and where the rest of the system is plugged in.