Can someone explain in plain english what the "Crossover" knob (50Hz -> 150Hz) on the back of my subwoofer does? And also the "Phase" switch (options are o and 180).

The crossover determines the frequency at which your subwoofer takes over providing sound from the rest of your speakers. Of course this should correspond to the frequency that your other speakers drop off their response due to their natural frequency response or what the receiver determines should be the crossover point.

The phase switch allows you to set your subwoofer to push out sound at the same time your other speakers are, or to be "in phase" with your other speakers. If your bass sounds shallow you should try the opposite setting from what you have and see how that sounds.

Ahhhhhhhhhhhhhhhh..... OK ok... gotcha. Thanks a bunch!

While X is 100%, totally and completely accurate as always, I would also suggest just "playing" with the controls a little. You are not going to hurt anything by setting the frequence to overlap your main speakers or if the subwoofer is technically slightly out of phase. This can be particularly true depending on the location of sub, the other speakers, what you are watching and most importantly, what sounds good to you! Of course we all want to hear the sound exactly the way the director intended, but shouldn't be scared to go with what you like, what sounds good in your room, system, etc.