So! My hum is history!
I had a feeling all along that it was a ground loop but had no way of defeating it because everything was plugged into the same strip.
Whilst I was waiting for PL and the dealer to come back I saw something in the local hifi store and thought “fuck it, let’s give it a go, if it doesn’t work I can always return it”
I bought 2 of these ifi silent power gnd defenders. They plug into your IEC port on the gear and then the power cable plugs into them. They supposedly drop the ground if they notice a loop, whilst keeping a safety ground, but do nothing if there is no ground loop.
GND Control The GND Defender (Ground) eliminates ground loops in your audio set-up that often cause audible speaker hum.
ifi-audio.com
Whilst not cheap at €100 each they have worked perfectly for me and are relatively speaking not that much when you consider they have killed the hum between a €1800 pre amp and €4000 power amp!
They also launched D.C. blocker in the same line which filters out any D.C. on the AC which is great for any gear where you have a beast of a transformer that’s physically humming/vibrating.
Your humming transformer is your quarterback. The DC Blocker (Direct Current Blocker) aims to reduce noisy physical transformer hum caused by DC on the mains. It’s designed for amplifiers that uses a mains power transformer in its power supply.
ifi-audio.com