The BTL line is our flagship woofer. Its whole design is based on and focused on being able to throw tons of power to something in a Bass Race or �Street Jam� situation. Options Available are as follows: P Chamfer: This is a chamfer on the end of the pole, it helps to bring the air down around inside of the motor and helps cool the coil better. It also helps relieve the pressure underneath the dustcap. We recommend at the bare minimum to get this option if you choose this series of woofers. Cool