Presence control strange behaviour

[GainFreak]

Hi,

I have this 2x6V6 amp with 6SL7 LTPI (with typical Marshall values, 50k feedback resistor from the 8 Ohm tap).
What I noticed is the Presence control behaves in a strange way unlike the one in my 4xEL34 amp and other amps I built.
Instead of only boosting the "presence" frequencies it sounds like it increases the volume as well. I swapped the OT leads but it started oscillating so I assume it's not positive feedback.
When presence control is at minimum and I disconnect it from the FB loop there's an audible sound change which is usually not noticeable .
Years ago I had similar problem with a 2xEL84 amp where this Presence "effect" was even more noticeable and I just left it without it.
I would appreciate if you have any ideas and or comments on what's going on.

[GainFreak]

It looks like the problem was the 100nF presence cap. Changing it to another type solved the problem. It's still strange though because it measures 100nF on my DMM, however not in the 200nF range but in the 2uF one. In the 200nf range it measures infinity.