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.
So your Hovland Music Groove cable connects the grounds between your tonearm and phono stage. Have you tried adding a ground wire between the phono and line stages? This can sometimes eliminate a hum like the one your are having, but the only way to know is to try it.
Try disconnecting the tonearm ground wire and see if it's better or worse.

Another thing to try is floating the ground on different pieces of eq. Floating the ground on my TT power supply helped in my system.
Thanks Bill_k and Sarcher30. I'll try those suggestions Wednesday and report back.

The thing that most puzzles me is the hum continuing after the externel phono stage is turned off. How can this be ? The question probably underlines my lack of knowledge in such matters.
In many designs the power switch does not switch the ground lead in the power line, so you can have ground loop hum even with the power switch set to "off".
Try moving the phono stage. It does not matter if it is on or off if another piece of equipment that is powered is causing an electrical field to influence your phono stage. The kind of gain phono stages produce have always been problematic.
I just went through this same issue after rearranging my system. I have to keep mine away from my premp and my new dac.
The dac is why the system was rearranged.
Today I only had time to run an earth to the pre-amp (no difference) and try to wrap the phono cables in an RFI shield (no difference). I'll try more as soon as I can.

Moving the phono stage away from the pre-amp (thanks Hevac1) is, relatively speaking, a fairly major change - so I'll do that last. This would probably anyway mean the Hum from the HP100 integrated phono has a different source to the separate phono (?).