The current lineup of Hartke bass combos are designed to work with the onboard speaker arrangement. The speakers that are included with your Hartke bass combo have been designed to match the internal amplifier for the most power and volume.
Some older models such as the Hartke LH5210 did include an additional speaker output, which a 8 or 16 ohm bass cabinet could be connected.
You can disconnect the internal speaker and use the combo's amplifier with another cabinet. Be careful to verify the impedances are compatible. Any adjustments you make to the combo or speaker can cause damage and void your warranty.