Interesting thread, and I'm sure we'd all love to hear the amp and the different clipping voicings. I don't build amps myself, but I would like to try to clear up some things about the effect of different capacitor types. AFAIK and ICBW, caps are not amplifying components, so I'm not sure they can generate harmonics unless maybe when driven beyond their voltage value...no?
I have read from several reputable sources that ceramic caps are more prone to resonant in an active circuit. It should also be noted that metalized-film caps have a smoother transition into the operating range (for lack of a better description), which may go hand in hand with less resonance. Therefore, if the "Treble" cap in a tone stack resonates, it will simply drive the amp section harder around it's resonance peak. That will generate more of some balance of even or odd harmonics from that peak depending on the amp section configuration.
Pentodes are said to generate more 3rd harmonics than tetrodes, and class AB amps cancel the generated even harmonics unless biased hot so 2nd harmonics are introduced. Therefore, a hot-biased AB tetrode power section will generate some 2nd, but less 3rd harmonics than a pentode power section, and a cold-biased AB pentode section won't generate any even harmonics, but more 3rd. The generated harmonics (of whichever mixture) will of course be stronger around the resonant peak of the "offending" cap. Is not that what is actually happening via different cap types rather than the caps themselves generating harmonics?
