1- Sprague hasn't made the orange drop capacitors since 1992, when they sold the line to Vishay, who then sold it to Cornell-Dubilier in 2012. The 'SBE' on the pic in post #5 indicates a Vishay part.
2- Parts bought from legitimate electronics distributors (DigiKey, Mouser, Arrow, AvNet, Newark, Tayda are some I've used) will reliably be within specification. Virtually all electronics parts are tested before shipping and any manufacturer who cannot ship almost 100% good product will be dropped by the distributors.
Parts bought off Amazon or eBay may be fine, or could be parts that failed testing, or are from pirate production runs, or counterfeit/misrepresented (like selling Shottkey diodes as germanium).
3- Capacitors can be rated in quality with measurements of their dielectric absorption, dissipation factor, and temperature coefficient. To greatly simplify, these are measurements of how the operation of real-world capacitors differs from that of an 'ideal' capacitor.
4- Ceramic disc caps were used initially because that was what was available when Fender began making basses.
5- In a typical passive bass circuit, the signal going through the capacitor is shunted to ground and the DA and DF of the cap don't affect the output signal at all. Capacitance is capacitance there.
6- Ceramic disc caps do have poor DA and DF, but more importantly for basses they often have very wide tolerance ranges (+20%/-80% is common), poor temperature coefficients and can be microphonic. So the capacitance value may not be what you want, it may change greatly when the stage lights hit you, and it may make noise when you jump off a speaker cabinet.
7- The orange drop caps are legitimately quite good capacitors but audio was not a consideration in their design and they have an outsized reputation among musicians. They were the first of the modern film capacitors to appear and were designed for motor controls, such as within air-conditioning units. That's why they have voltage ratings of 100-600 volts, why they're so large.
In a standard passive high-cut RC filter like in a bass, any of the film capacitors (mylar/polyester, metalized polyester, polystyrene, and polypropylene) will give you the same performance and are better than a ceramic disc type. Orange drops are polypropylene (which has the best DA and DF and are inherently the largest and most expensive to produce) but a 25v rating would be more suitable.
8- There seem to be a lot of people making money selling myths, ju-ju, and magic beans to musicians. With a little bit of ju-ju and repackaging, they can sell a part bought for $1.38 ea in hundreds for $6.53 ea (actual example) to musicians who don't know any better. It's our own fault when we reject science and fall for the scams.
