Active biasing of capacitors in a loudspeaker crossover was invented by Gregg Timbers of JBL/Harman. It is used in the Everest D66000 for an example, applying batteries as mentioned above. In the newer Everest D67000 the bias voltage is achieved via diodes rectifying the incoming voltage from the amplifier which charges an electrolytic capacitor. The advantage to this approach is that the bias voltage always will be higher than the incoming voltage, and there no need to replace batteries.
JBL Everest D66000 XO schematicActive biasing of capacitors places a bias voltage to a midpoint between two capacitors relative to ground. The voltage can be applied with a battery, or a series connection of batteries to achieve a higer bias voltage or as described above. Th sonic benefit form active biasing is only achieved as long as the bias voltage is higher than the incoming AC voltage from the amplifier, that being said 27VDC (3 ea 9V batteries in series) is certainly high enough for even very loud playback.
Good Listening
Peter