Look inside the chassis of any Fender blackface reverb combo that
still has all its original coupling caps. Take a look at that el-cheapo
reddish .02uF coupling cap coming off the vibrato channel's 2nd
gain stage (V2B). Does anyone seriously think that this particular
capacitor was chosen with tone or performance in mind? Notice
that it is the exact same cap as the .02uF used in the trem oscillator
circuit. Do you think it's likely (I sure do) that, during this era,
Fender's amp manufacturing made the decision to use ceramic caps
for values .02uF or less and film caps for values .022uF and
higher? From experience, I know that just about any type of film
cap, orange drop, Xicon, or whatever, is going to work and sound
A LOT BETTER than that awful (and usually microphonic) .02
ceramic cap, once it's replaced. So I have a very hard time
believing the Leo Fender specified that this cap be ceramic in order
for his amps to sound magical.

the specific ceramic cap I'm talking about is right in the main
signal path of the 2nd (reverb/vibrato) channel. Whenever I restore
a Fender reverb amp, I make a point of always replacing that
sucker with a decent quality film cap, unless the customer has
requested that I keep things as stock as possible.

 Good point! One of my SF Bassman heads came with those
cheap ceramic caps all over the place! (They must have run out of
the better quality polyester caps... or somebody screwed up
ordering the parts and someone said just use them up!)