Had some time to try this & here are results, observations & general thoughts.
The amp is my Maxim Triple Drive (basically a HoSo56 for the "clean" channel, cascaded into a triode gain/CF (Dumble "ripper"-type circuit) for OD.
I tried a wide selection of different types of caps (ceramic paper, ceramic plastic dipped & silver mica dipped) and values (68p, 100p, 120p, 180p, 220p, 250p, 330p, 390p, 470p, 590p).
The effect of the cap is subtle, but significant to my ears, & "fixes" a problem that can be difficult narrow down in these high(er) gain circuits that generate most of their distortion in the preamp (vs power amp distortion). They can tend to have a certain "buzziness", and many solutions have been tried, some working very well (smoothing caps across preamp plate resistors, high capacitance sheilded cable, high value grid stoppers, etc) & others, not so well.
In my amp, I had fine tuned the circuit to as close to "perfection" as I could, but there was a certain "something" that was still not quite pleasing to my ears on certain notes, especially on higher gain solo notes in the upper register. It was almost akin to & sounded similar to what is commonly called "cone cry" in some speakers, but not quite the same, & was present equally with different speakers, both new & old & different types & manufacturers. I have heard some builders refer to it a an "artifact" riding on the note.....I refer to it as "hash".
These caps ALL had an effect on removing that bit of "hash" & giving the finishing touch to an already "perfect" amp, but some values had a more pleasing effect than others.
One interesting thing is that the value of the cap has a slightly different effect on frequency response when used in this location (PI input side plate resistor) vs the normal use across the preamp Rp.
In the preamp, the cap simply removes more high frequencies as the value of the cap is increased (blanket on the speaker effect).
In this application, the cap value seems to actually shift the frequency response (not simply add or subtract higher frequencies). The effect seemed to be on the upper mids, and affected the "vocal" qualities of the amps "voice" (if that makes any sense). The high frequencies were still there when the cap was added, but they were "smoother" & "clearer" (less unwanted artifacts/noise on top of the notes), and increasing or decreasing the cap value (within reason) didn't have as much effect on this desired result (smoother high frequencies) as it did in changing the upper-mid "voice" of the amp.
I tried all the above mentioned values & types, settling on 180p as the best value (to my ears & for my amp). None of the values were "bad", just "different" & not what I was wanting to hear from this particular amp.
I had both dipped & non-dipped (paper covered) ceramics in the 180p, & finally settled on the plastic dipped unit as the "best".
Again, this is a subtle effect (to my ears), but is quite useful in putting the "icing on the cake" of an already good sounding amp.
I like it & the 180p is staying in the amp......Thanks Tubenit, for the new tweak! 
$0.02
Geezer
Thanks for your extensive analysis and results.
It may cure what ills one of my high gain amps.
MG
“Failure is not an option”
