80hz crossover for your mains will split the signal going to your mains in the following way:
Above 80hz --> Mains
Below 80hz --> Sub
Of course it is not a brickwall, so there will be some overlap.
The above has nothing to do with content in a surround mix programmed specifically for the subwoofer, which is the only thing influenced by the LPF of the LFE.
The LPF of the LFE is only referring to the content that is programmed into the LFE channel (the ".1" in 5.1) - content specifically directed to the subwoofer only.
If you set the LPF (low pass filter) of the LFE (low frequency effect) to 80hz, you are saying that you don't want anything above 80hz found in the LFE signal to be sent to your subwoofer (again, not a brickwall). An LFE signal may contain content up to 120hz, so in this case you could potentially be cutting out the 80-120 hz content, provided any was there to begin with.
The two settings have no influence on one another.
Originally Posted by Mongao
Please, why ?