Low rumble when connected to SVS SB-1000 subwoofer.

I connected an SVS SB-1000 subwoofer to a Powernode N330 and there is a constant low volume rumble, sort of like tires rolling on asphalt.  The rumble doesn't appear to increase when I turn the volume up - it seems to be a constant unchanging low volume background noise.  I've tried swapping out the RCA cable between the two to no avail, and put both the Powernode & subwoofer on the same outlet (actually a power strip), and checked that the RCA cable and power cables are not running in parallel in an attempt to address a possible ground loop.  I'm stymied.  Any ideas on what to do next?