I assume you have the sub in Auto mode where it is looking for a signal to alert it to come in. You say you have to turn the sub on manually, how do you do that? In Auto the sub is looking for any signal which is less than the crossover frequency you have selected on the 2160. If the volume on the sub is set low you may not see much effect. Try about 50% for starters. Your crossover point depends on the capability of your main speakers so you should set your based on that (55hz). Also make sure the crossover selector switch on the 2160 is not in bypass because if it is and the crossover knob on the sub is set to low it won't get much action for most music. Bear in mind that your main speakers should cover most of the musical range and even though they are rated down to 55hz you should cross them over at about 80hz to allow a smooth transition between the mains and the sub. If you are using the Xover switch on the 2160 and the sub is in Auto make sure the Xover knob on the sub is all the way up so the sub doesn't try to interfere with the signal from the 2160.