No details online about how those speakers are connected.
If the sats and sub are connected to the control box with speaker wire that means that the speakers are powered from the box and you would need an adapter to connect a self powered sub
If the subwoofer has a power cord and the sats are connected with speaker wire to the sub then you can't change the sub because it contains the power for itself and the sats.
Don't worry about the power rating of the sub. It won't affect your result. Don't get a sub that is band pass loaded. This means that the speaker is inside the cabinet and the only sound you get is through a port opening. Lousy sound and would not work at all with such small sats.
If you are using a soundbar and add a separate self powered sub you probably don't need the control box or sats anyway.