An update, for anyone interested...
I tore into things last night and found the problem: an AC extension cord with a switched box at the end. I'm using a seperate extension cord, with an on/off switch, for my monitors, running that cord into an APC battery back-up/surge protector box. When I replaced the extension cord, the hum dropped down to a soft hiss, which I can't hear unless I place my ear against the speakers. Problem solved--and I thank all of you for your helpful suggestions. Ah, the sweet sounds of silence, especially late at night when every click or hum throughout the house seems huge.
J.