LFE is Low Frequency Effects. It is generally the signal that goes to the .1 in 5.1 and 7.1 systems. It is encoded as a separate track in most movies and is targeted to the subwoofer channel. In the 990 manual on page 29 is a pretty good description of how the 990 deals with LFE and also how to set your subwoofer low pass filter (crossover) based on the size and capability of your main speakers. A setting of 80 hz is generally a good place to start even if you have large speakers. The thought is that since lower frequencies require so much more power to reproduce, a powered sub is a better device for producing them and it puts less strain on your amplifier since it is doing less work. That is why the 990 will generally set your main speakers to small even if they are very large when you run the auto setup program. It takes a real bass hog speaker to get the 990 to set it to large.