These things:

FMOD Crossover Pair 70 Hz Low Pass
What is an FMOD™? The FMOD™ is an active crossover simulator. It is better than a much more expensive electronic crossover that requires power to operate. FMODs™ are packaged in pairs and can be combined with other FMODs™ to obtain bandpass ranges or with other types of crossovers to change the slope and crossover frequency point. FMODs™ feature a 12 dB per octave crossover slope and are compatible with all RCA type input amps...simply place the FMOD™ in-line with the patch cable running between the source (head unit) and the amplifier. 24kt gold over brass construction, 10 volts maximum RMS input. One-year manfacturer warranty. Made in the U.S.A.