The low pass limits the frequencies the sub will play. Adjust as necessary to make it sound seamless with the speakers. If you go too high, the direction of the sound will be obvious. You'll be able to hear sound from exactly in the direction of the sub. If you go too low, you will miss some of the frequencies just below what the speakers can play. You might want to start at around 60 to 80 Hz.

The phase switch is a more complex topic, but suffice it to say...leave the switch in the position that provides stronger bass if you can hear a difference between the two. It serves to correct a mismatch of the speakers and the sub.