[Texasrocker]

No prob Doug! Thanks for asking!! After having restored well over 250 various brands vintage radios in the many past yrs from 1920s trfs-50s superhets, I have a very good understanding of why to replace "all" caps in vintage radios! The "I dunno" as you wanted me to reply to is this. I dont understand ,just as you, why the professional-musicians have created a demand for pre-used vintage caps among the guitar-gods except for Im talking major Recording Stars here than can afford to purchase "fine vintage amps & guitars).... for this explanation that you asked for, except to "splain" it this way. I really know why the real "musicians & their techs" do, but Im not sure the radio-techs here will??!!! You see Doug, the used vintage black-beautie type caps I was referring to be reused again, are in applications such as "inside the guitars bodies- from the pickups assys/ or vintage amps used on the vol-tone controls only",,, very ( low-voltage) situations! Most vintage radio caps are "trash" indeed!
Except for a very-few of the vintage black-beauties and/or any vintage wax-caps, that "never" had a resistance load across them in such areas as at the loop-antenna-to chassis input connectors of vintage radios,or other vintage electronics from that era etc such as a mid-50s McIntosh C-8 preamp was "loaded" with sprague BBs in a low voltage situation also just as a example. I stripped several very good re-usable vintage BB & wax caps before and tested them!
As far as reusing any previously used vintage Sprague BB ,or any brand wax cap in a "amplifier", those limited "very few" uf candidates are tested fully before re-installing them into amplifiers only used in Recording Studios "house vintage guitar amplifiers" where "tone" is paramount in the ears of the audiophiles, and "yes", those guys DO exist beyond the vintage radio restorers world indeed!Those guys can tell you if a 1959 Fender Bassman amplifier has ever been "re-capped" before just by listening to it, before ever pulling the chassis out ! These "real" vintage type caps are indeed in demand for "testing" guitar tones,and replacing in vintage "highly-rare" Vintage collectible guitars that someone molested in the past by installing new caps and/or the orig guitar pickups.., and the guys that "try them", know what they are doing with such vintage items from the get-go!! Many vintage guitars & amps have been highly-molested before thru the yrs just as vintage radios.The highly-professional ( Recording Artists signed to Major Rec Labels)vintage guitar & and amp "techs" really DO know more than you give then credit for!! They definately know better than to just "chunk-in" various vintage BB & wax "ebay find" caps into a guitar or amplifier! Being that some of the orig 1959 Les Paul & 57 Fender Strats Guitars now sell upwards of $50,000. each, and the only thing missing is a "real- genuine" 50s era vintage wax or genuine vintage sprague BB cap under the pickguard, and you bet Doug, these vintage caps are sometimes in demand to be used to bring the instruments back to "original"! When someone is paying up to $50 Gs for a vintage guitar,( or vintage amplifiers) they "do" pull the covers and look closely, and they know the difference between the new-type wax caps being mfd today & the vintage! Its kinda the same-thing as the guys that "re-stuff" the philco-bakelite blocks !! Many of the vintage sealed-type BB caps are still usable to a certain "group" that knows how & what to expect from their service & purposes!! Being that most of these used vintage caps are now re-used only in tone-controls appliques, ( low voltage) situations, by folks that know HOW to restore vintage-tone to vintage guitars & amps, you bet, a few of these vintage caps are still in demand! As a pro-touring musician for many yrs myself, even I would know where to "sub-only" any used wax or BB cap in ANY acceptible low-voltage situation! So in retrospect, I "do-no" why guys like SRV( had) & BF Gibbons( guitarist) of ZZ Top still have "use" for such items!!..and they arent "scared" to experiment at all! Splained? If not feel free to rsvp! until then "cheers"!! Its happy hour here & your always welcome for further "splainin" my posts anytime!! Thanks again for your "interest" Doug!! TEX