This post isn't about that kind of capacitor. Yes you are right in the fact that a capacitor will give a "boost" to an amp for a short period of time, but those caps are usually much larger (approx. 20,000 times larger) than the ones that you would use for a -6dB low-pass crossover.
Yes there is a way that you can limit levels going to certain speakers, but if there is a fader control that is the easiest way.