Linn LP 12 hum not coming from Bryton phono stage.

Have you noticed whether the intensity of the hum changes as you listen to an LP, from outer grooves to innermost grooves?  If not, you might do that experiment and report back.  Absent a defect in your current power supply, I doubt that the PS itself is the problem.  Or to put it another way, don't spend the money on a new PS solely in order to cure this problem.

What I was getting at is whether the hum is due to a direct interaction between the cartridge and the TT motor.  One manifestation of that would be that the hum is loudest when the cartridge is nearest to the motor.

I'd start by checking the headshell wires of the K9.

Sometimes they can get too close to other pins. If so you could try to move them with a small non-metal implement like a toothpick etc. Just to see what happens to the hum.

You could also check the mains cable by moving it to see if that has an effect. Loose connections are notorious for causing hum issues.

As for the Valhalla board, I would not recommend an examination. It carries lethal voltages and you would also risk disturbing the suspension. Without the correct suspension setup, the sound of an LP12 collapses pitifully.

It might well be possible that the thin earth lead which grounds the sub-chassis may have strayed a little and is now picking up something from one of the board's high voltage components.

In this case you will be better off getting it checked by someone who has the required setup jig. They'd be able to take a closer look and give it a service and oil change etc at the same time.

I'm also far from convinced that the Linn belongs on a wall shelf. It's suspension can cope remarkably well with vertical resonances sat on a table, but not so well on a shelf with resonances coming from a wall.

However this is what I found some 15 - 20 years ago. Linn may have changed the suspension components a few times since. You should definitely consider ringing a dealer or maybe ringing Linn themselves. Sending an email certainly won't hurt either.

Turntable hum is the nightmare of every vinyl enthusiast, so the best of luck in getting it fixed