We recommend turning up the volume level controls all the way on both subwoofers. Listen to a piece of music you are familiar with that has intensive basses.
Alternatively, you can also use the test sound of the receiver/amplifier. Now ask a second person to lower and raise the volume of the one subwoofer and then that of the other subwoofer. As a small aid, you can disconnect the normal speakers so that only the subwoofers are running. Both subwoofers are correctly adjusted when you have the feeling that both are equally loud.
Alternatively, you can also use the test sound of the receiver/amplifier. Now ask a second person to lower and raise the volume of the one subwoofer and then that of the other subwoofer. As a small aid, you can disconnect the normal speakers so that only the subwoofers are running. Both subwoofers are correctly adjusted when you have the feeling that both are equally loud.