I think what you're looking for are bass blockers......
Bass blockers ARE capacitors, just a different type/size used to a different purpose. Capacitors are devices that store energy and release it in bursts. The obvious use of this is what you mentioned about keeping juice to your amps, giving them a nearby reserve or power.
caps also by function pass a/c but block dc, by using a cap of the right spec you can filter low end signals as they will be seen as d/c by the capacitor. This is opposite the funcion of an inductor, which will block high signals because as the voltage increases, so does inductive resistance, when the resistance is too high, voltage will no longer pass through.
(type/size,value of these components are what determine db slope and crossover frequencies)
I'm pretty sure I typed all this right, but I'm kinda tired so..
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*Rides*
-1999 Ford Contour SE Sport "Beater"
-1987 Firebird Formula Dragster (not even approximately close to stock)
