Now that I’ve got a subwoofer component in my system, I hear the hum even at low volume, but only when I get close to the speakers. Otherwise, I don’t know it’s there; that's what you said.
"Otherwise you don't even know it's there".
You do not have a "hum hum" problem, but one that goes with that combination of components; that you only hear when you get close to the speakers; that you did not hear before you got the "subwoofer".
You can either change components, enjoy the music, or spend the rest of your life trying to fix a problem that you didn't even know you had until you got the subwoofer.