I think its how the different types of capacitors affect the tone when you put the tone control at, lets say, 3 or 7, and not much at fully open (10) or fully closed (0).
That's more the pot than the cap, but capacitors do have what's called an equivalent series resistance which does load the pickup and is different for electrolytics (higher) than mica caps, HOWEVER, at the voltages passive pickups operate, it's just not going to make a difference, even with the highest impedance/gain amp inputs. It's easier to induce a voltage across a higher resistance, hence, in general, the higher the coil impedance of the pickup, the greater the signal output, and also why piezo pickups sound like crap unless you have a dedicated super high impedance input for them. That's also why tube amps sound more "open" than most s/s; tube input stage impedance is way higher than that of transistors.
Different types sound (ok just a little) different at those situations. That's what I believe.
But the main reason I prefer to use a well known brand cap instead of a generic one is the quality of the part. It's like you buy a new bass that has those dreadful microscopic pots, and you change them with CTS for maximum reliability.
Mica caps aren't 'off brand,' they're small and cheap because they are made to exactly what a tone control calls for, small-signal filtering. If you want Sprauge Orange Drops, put them where they'll do some good: in an amp.
