"I discovered that if I slightly loosen the coax cable going into my cable box, the hum disappears. I’m still getting a signal, so this appears to be a reasonable workaround. I’m wondering if there are cables available that specifically address this problem."

You can try this isolator as one possible solution...

http://www.amazon.com/Viewsonics-VSIS-EU...d+loop+isolator

"My backup plan was to buy a new Blu-ray player with two HDMI outputs, and run one into the 975 and one into the TV (my current player is an old Sony BDP-S550, with all firmware updates). Maybe a newer player would eliminate the handshake issue anyway. However, I’d rather wait until 4k players become available."

Bear in mind, you will need a processor that is compatible with 4k, HDCP 2.2, when you plan on using a HDCP 2.2 source device. The 975 is not HDCP 2.2.