And its got to be placed somewhere
(I don't know, I would have to see how the pot is connected. I assume you have just the one pot on that board. Valve heater is common to both, so hum bucking was my best guess)
Done somewhat in haste so here's some more details:
1. AN Soro is indeed the Audio Note Soro amp
2. The picture shows the input board and the pot is there in the bottom right hand corner
3. The amp is not mine and I am diagnosing at distance for a friend. If I could see the amp I could work it out in seconds....
4. No pot is shown in any Audio note schematics
5. So I'm really looking for someone with Soro experience to be able to tell me
Can the schematics be posted here?
If you look at the photo link of the Soro in post #8 you will find there is no pot on that board and that since the Oto uses the same input board, also with no pot, it may be special. Maybe if you can get your associate to take a decent photo of the board and the area of the pot you could get a decent answer. It could be a different revision than most and perhaps someone, even you, living in the UK, could just phone AN and ask.
All of the guesses in the world won't get you any closer to an answer.