Hunting down a HUM

Most of us have been through this at times. This one is presently defeating me.
When I turn off my external phono stage the hum persists - undiminished. Phono lead (Hovland MusicGroove) is earthed both ends. Always perfect silence on all other line inputs.

Until recently I was using the integrated mc valve phono stage in my Hovland HP100. (RIP Hovland) It hummed but not too intrusively and I put it down to the valves.
Last week I resurrected my EAR834P which had been modified to their top spec by Great Northern Sound Co (RIP GNSC). The hum was noticeably quieter but still evident - and still total silence on all other inputs. Again I put it down to the valves.

I only found out the hum persisted because I could turn off the EAR before the HP100 (yes - I did wait a while...)

In case relevant - VPI SSM Ref deck with 10.5i arm. Apart from the Hovland phono cable my system's wiring loom is exclusively Audience. (Au24e & SE interconnects. Powerchords all 'e' except the Au24 feeding the Audience AR TSS2 power conditioner). Finally - I live in deep countryside where RFI is nothing like city levels. I've never found it a problem (?... before..)

Any thoughts on causes &or elimination processes very gratefully received.