To isolate the hum, I unplugged all AC power cables except the 950 and the subwoofer. I still heard the hum. Then I disconnected the cables from the cable box to the TV (TV is connected to the 950). The hum went away!

So the hum must be from the cable TV line and I will purchase a coax ground breaker.

I was surprised that I could not find a coax ground breaker on the Radio Shack web site. I found the following elsewhere. Are they the type of product I want? Of course, I am open to other suggestions.
http://www.accwh.com/59564-Coaxial-Ground
https://www.amazon.com/TII-220-Ground-Isolator-applications/dp/B0070Q6URO
https://www.amazon.com/Viewsonics-VSIS-E...ZWYTAWPGC5XN20T

Thanks, everyone, for the suggestions.