Looks like that amp only has 1 output jack. See if you can find out what impedance the amp can handle, as well as the impedance of the speaker in the combo. If the amp can handle a 4 ohm load and the speaker is 8 ohms, you should be able to connect another 1x15 ( preferably matching the cab in the combo) for more volume. However, if the amp handles 4 ohms and the speaker is also 4 ohms, all bets are off. At that point, you would have to change the speaker out for an appropriate 8 ohm driver to add another 8 ohm cab. Then you may have to change the crossover in the combo as well.
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