Disable HDMI On S905x MeCool M8sPro+

I would like to be able to disable HDMI output on my S905x MeCool M8s Pro+
Ideally this box would be used as a headless music player which autostarts an internet radio channel and plays to the built-in analog output
Much of this is working EXCEPT there’s an awful hum on the analog output unless an HDMI cable is connected to a valid “sink” device (tv or amplifier)
I’m hoping the hum wil be supressed (and analog audio will continue to be available) if HDMI is switched off - perhaps by forcing CVBS output

I’ve tried

  • fw_setenv outputmode 576cvbs
  • echo 0 > /sys/class/amhdmitx/amhdmitx0/hdmi_init
  • userdata/dispcap with entries like 480cvbs
  • userdata/cvbs_fallback with entries like 480cvbs

Nothing seems to work - the hum persists

Is there a way to force HDMI off ?
Currently using an external USB soundcard, which of works … but adds HW, power consumption and complexity that I’d rather avoid

thanks in advance