This is such an easy thing to google... http://www.alkeng.com/ap12_xo.html
The AP12-xx series networks are designed to operate with the Klipsch K33 woofer which represents an impedance of 6 Ohms in series with 1 mHy voice coil inductance.
AP12-500 This one is intended for the Belle Klipsch or the La Scala. The crossover is at 500 Hz.
AP12-600 This one is intended for the Belle Klipsch or the La Scala using squawker drivers or horns that will not safely go down to 500 Hz.