The whole issue of capacitor opinions and myths has frustrated me for about a year now; I finally buckled down and looked into some actual primary research on the subject, as well as a number of secondary research papers, rather than simply tertiary sources, opinion, and myth.
If I am understanding what I have read, ceramic capacitors, because of their construction, are microphonic, and, because of their mechanical/electrochemical properties, they produce odd harmonics under voltage. The first is simply explained: the ceramic disk cap is composed of two plates of material (separated by an elastic dielectric material) which are subject to resonating and picking up vibrations, and this makes them microphonic.
They also have a very high dielectric constant, which makes them noisy - they are made of inherently piezoelectric materials; send voltage through this device, and problems immediately start to crop up (because of the construction and physical properties of the device's components (and their interrelationships, of course)) (the plates resonate b/c of the voltage fluctuation; this attracts and repels the plates, and this produces, of course, noise) - though as I understand it, these effects are usually thought to be most noticeable in high-voltage circuits, but are still present at any voltage... and moreover (this is a separate issue, note), some caps (not just ceramics) show very sharp deviations in capacitance with variations in applied voltages, and this causes harmonic distortion.
This last point is important to guitar tone caps: I have seen graphs showing that not only very high voltage influences cap performance, but that very LOW voltages -like those from a guitar pickup- will cause distortion as well, though the warped, s-shaped curve -which differs for different dielectrics, of course- is opposite for low and for high applied voltages. I will try to be clearer: at very low voltages, some dielectrics, and here speaking of caps in general and not specifically of ceramic disks, will show marked swings in capacitance under varying voltages (AFAICT a "perfect" or non-varying dielectric does not exist in the physical world, though this variable capacitance curve can be and is very regular and shallow with some dielectrics and construction methods). At low voltages, these "high-variation" dielectrics show quite a bit higher-than-rated capacitance, and at high voltage this curve reverses. This swing from high-capacitance to low-capacitance can be quite sharp in these dielectrics, especially at low voltages.
Ceramic disks have been shown to produce harmonics in the odd series (I have not seen an actual explanation for this but it seems to me that the odd harmonics are produced b/c the plates are equal (to fair precision) and opposed (again, with pretty fair precision), cancelling out even harmonics, but this is ignorance and speculation on my part). In any case, just b/c of their microphonic properties, I will be avoiding ceramic disc caps in the audio path (except maybe for the NFB path in my amp; I might want to see if applying the "extra" odd-series harmonics as negative feedback to the P.I. tube will attenuate the "naturally-produced" odd harmonics).
MLCCs, AFAICT, are only microphonic with a bias voltage applied to them (although they are piezoelectric (all caps are), and thus show additional noise (that is, additional to the basic "thermal" or Johnson–Nyquist noise and the schottky (or "shot") noise (which is present in all conducting materials and is due to quantum fluctuations)). If you want a tiny cheap cap in a guitar circuit, an MLCC might be okay.
Opinions seem to be divided upon the audio qualities of silver mica caps in both the guitar and audiophile communities. Some like their treble response and some find it harsh. I can't find a good explanation for this but I have seen it suggested that their low noise in the upper registers makes them sound both clear/digital and harsh/digital. The pink noise produced by (the wonderfully inefficient) carbon comp resistor is similar to this phenomenon, I gather. It is speculated that it is this quality which makes which makes carbon comps seem seem warmer and more pleasant than resistors producing less noise or equal distributions of noise (white noise), but this is psychoacoustics, not engineering... but I digress.
Will I pay $20.00 for a NOS PIO cap? well, um, no. For one thing, if it hasn't been in a circuit with voltage running through it, its value is likely to be off to a greater degree than if working, and it is much more likely to be leaky. NOS parts can look lovely but are quite expensive; there are modern parts which perform quite well, even if what you are looking for is a very inefficient part. Some people like the sound of class-A amps (I am among them, but chose to go the route of a pseudo-SET 3-way conversion mod combo to my to my push-pull EL-84 amp, which is quite complicated, but a real blast to do, and allows me infinite variation among 24 distinct power-section circuit topologies (running in four modes), ranging from pentode push-pull to single-ended triode amp (either tube, and yes, they bias right and get the full signal from the P.I. tube)... and I don't need an air-gap tranny!), and they are extremely inefficient from an engineering perspective, but generate even harmonics much better than push-pull topologies (push-pull topologies cancel out even-series harmonics (if well-balanced), as I said before - and as most of you surely know).
Poly caps seem best to me, without going into PIOs. They have good physical and electrical qualities. Remember I am a tyro. Upon the subject of PIOs and such I really need more info. can anyone help?
The cap time constant question still bothers me; I can't find an answer to it. Ditto plaintive "can anyone help?".
Anyway, sorry if I seemed snotty before; I don't mean to be brash; I do really want to know all this stuff, and I am an almost complete novice at it. In addition, I cannot do math to save my life. (Despite that, I get by.)
For the record, my cables run circa 10-15 pF/foot + ca. 10 pF/connector. They weren't "expensive", in that I have like, 60 of them, and have for like, ever, so no added expense was necessary to simply plug cables together - and it was a heck of a lot easier than learning to solder, too (I would have never done that without the above challenge to solder a cap into a cable ("cheaper" my a$$! -The soldering iron and solder were cheap, but not free)). ...And I only had two capacitors at the time, one in my CE-22.
Also for the record, and re the treble bleed issue (if not the actual wiring considerations), and if anyone cares (as if, Felix!), I have wired up a varicap as a diagnostic device, but have not used it for my "'perfect' treble bleed cap value" determination yet (ah, psy parties and anime... and legal troubles, oh my!), but I have used it as a diagnostic tool on my amp's smaller cap values... it works. Update soon on treble bypass values as Felix prefers and as determined by NOS varicap (2 for $5.00; I've paid twice that for one modern wirewound resistor!). I have two air-gap variable caps coming which are of higher value and wider variation (my present caps only go from 20-386 pF; these are 20-680 pF or so) -and which can take the 650V I need them to! ...This sort of component could be a quite effective, if trivial, tone control if applied between the guitar out and the amp.
Edit: I should have said "it would have been a while before I first picked up a soldering iron"; I would have been modding my amp before too many more years had passed, probably. Still, the push was appreciated.
Edit: I should have said that push-pull topologies cancel even harmonics generated in that stage.
