The Kickback KB12/KB15 amplifier was designed to work with its internal speaker. The internal speaker is connected to the 1/4" jack on the rear of the amplifier.
Because of the high power levels and low frequency content of the signal generated by the Kickback amplifier, we recommend the use of the provided internal speaker only. If you do wish to connect an external speaker, use only appropriately rated 4 ohm speaker cabinets that are specifically designed for bass instruments.
If you want to connect an external cabinet, you will have to disconnect the internal speaker.