Well, as surprising as it may seem, the only explanation that suggests itself is that the HTS3600's digital circuits (which include a microprocessor) are coupling digital noise onto some of its outputs.
You could pretty much confirm that by bringing a battery-powered portable AM transistor radio near the power station, with its dial tuned to the low end of the band (near 540kHz), while nothing is plugged into the power station. You'll probably hear an increase in the volume of static or background noise on the radio.
Regards,
-- Al