"LFE" is the Low Frequency Effects in a movie DVD. When you say you have a 5.1 surround system, the .1 refers to the LFE channel. When you tell a receiver that you have a sub, it directs the LFE channel AND all info below your set crossover point of your 5 other speakers to the sub. All receivers are different, so you would need to see if your receiver will send the .1 info to the left and right speakers if "no sub" is selected in the setup menu. If a disc has no LFE track, your ft speakers receive a full signal when set to large. I have the RF-7's which are rated to 32 Hz. There is quite a difference in listening to 2 channel music with them set to large as opposed to setting them to small and letting the sub do what it was designed to do, mainly handle the lower octaves. It puts less strain on your amp and speakers when the sub is on equaling better sound IMO.