Thank you for the advice about NOT needing any amplifier, you were right!
I removed the old Antronix VRA900 amplifier. I installed a -70 dB moCA POE filter, the PPC 2520A GLP-1G70CWWS ($8 at Amazon), an $8 Holland GHS-3PRO-M 3-way splitter, and a bunch of 75 Ohm terminators on all the unused...