LFE is the .1 of the 5.1 or 7.1 soundtrack and can have content as high as 120Hz in it. Because of this, the subwoofer's built in crossover should be disabled or adjusted to the highest frequency do it will reproduce the higher frequencies when the receiver sends the LFE signal to the subwoofer output. We have talked about this in great length at our former home.
The subwoofer crossover frequency in the receiver is to adjust the crossovers for the main, center and surround speakers so that bass which is in their recorded channels in the soundtrack can be send to the subwoofer (which is also getting the LFE signal) in order to ensure the smaller HT speakers are not attempting to produce deep bass and that the bass gets reproduced by something when those speakers cannot. This is the "bass management" part of bass management. Setting all the speakers to the same crossover frequency makes sense for a non-ideal setup where you are not 100% in control of the acoustics and the room. It is an easy way to get past some of the gotchas associated with bass management. However, if you can adjust your room, speakers, acoustics, and measure their performance with a decent measurement setup, it isn't necessary to set the crossover to the same frequencies.
If you set the crossover for each speaker type to be most appropriate, then you get the highest fidelity, but it can be tricky to make it perfect. If you set the crossover to the same frequency for every channel, it the bass will be at least consistent even if it isn't the best possible sound for each speaker.
So, setting the channel crossovers is separate from setting the LFE crossover - they are two different things.
When you set the channel crossovers, the bass below the crossover point for each channel is sent to a Subwoofer mixer which combines those signals together into one signal which is sent to the subwoofer output. It also combines the LFE channel into that output, so you are in essence sending up to 8 channels of bass to one output. You have the bass for each of the main speakers (LF, LR, C, RSS, LSS, RRS, LRS) plus the signal in the LFE channel all going to the same speaker - which can be very difficult to reproduce and which is why an excellent subwoofer is clearly audibly superior to one of those cheap small noise makers.
In my rig I have these signals being combined and sent to my subwoofer(s):
Left & Right Stereo = 40Hz and lower
Center = 50Hz and lower
All surround channels - 60Hz and lower
LFE = entire channel at 120Hz and down
My preamp has the ability to set a crossover on the LFE channel and redirect the higher frequencies to my main speakers, but I choose to not use that feature.
