The hum is so loud that if I turn the volume to even the first notch it feels like the speaker might blow!
If no XLR cable is plugged in then the hum disappears, and if I plug-in an XLR to minijack cable (which probably has no grounding), there is also no hum. But if I use other XLR cables, the hum returns immediately, even if they are not plugged in.
@filbert Sounds like this could be a shell grounding issue
XLR connectors have a normal ground connection on pin 1 but they also have optional grounding of the outer shell. this due to the variety of signal types xlr (and other d series panel mount connectors) are used for as some use cases and signal types require shell grounding whilst others dont
to help diagnose this better can you tell me what audio source is connected to the other end of the xlr and can you open the XLR connector at each end of the XLR and share photos of how the wiring has been terminated