This is super strange.. what is going on here?

[g1]

it’s a subtle control.. seems to do most in the last 30%

What is the value of the resistor I have circled in red?

[Indyryder02]

Yes. Duh! The obvious first place to look.. ♂️
I’d check everything and just glanced at it wrong.
100 ohm not the 100k it needed to be. This also fixed the backwards operation of the pot.
Thank you truly for pointing out the obvious!
It’s the first thing to overlook always!
And I definitely did this time!
second pair of eyes is always handy!
many thanks!

[g1]

100 ohm not the 100k it needed to be. This also fixed the backwards operation of the pot.

Glad to hear that was the fix. I had suspected there might be a low value or short effectively bypassing the treble section.
Another thing I notice is those 100K plate resistors are a very non-standard color code scheme (the brown-black-yellow-red-white ones). They must be the Dale/Vishay military type as shown in the datasheet attached. It would be a lot better if everyone would just stick to the standard.