Guitar Tube Amp RC Filter frequency fluctuation
A low pass filter is pretty much defined by the R and the C value: -

\$f_C = \dfrac{1}{2\pi R C}\$

The same applies for a simple RC high pass filter and the steepness of the transition from "pas" to "reject" frequencies is very shallow: -



Even if the cut-off frequency did fluctuate by (say) 1% (and likely very slowly due to warming and cooling effects of the capacitor) the perceived change in tone would be minimal over that long period of time (at least several minutes.

Think about a slow 1dB change in volume - this could occur in a space of 2 seconds and you just wouldn't notice it let alone over a period greater than several minutes.

And there is certainly no dynamic effect of tone shifting due to the way the guitarist picks or strums the strings.

I don't think looking at CR filter changes is going to yield results that you think. I play and record guitar and have done so over many years. I do all sorts of things to the recorded guitar sound from basic equalization (messing with tone controls) to complicated effects and the least clearly obvious things that effect the sound of a guitar is movement of tone unless it is something dramatic like the wah-wah effect.