Dana, "filter out unwanted hum" means that that may be the only thing found above about 80Hz in the LFE channel. The LFE channel extends to 120Hz, but in practice, little or nothing above about 80Hz is mixed into it. The question of whether to set the LFE filter at 80Hz or 120Hz is of little significance(I use 80Hz)either way, and as you now understand, this has nothing to do with the crossovers between the speakers and the sub on the regular channels.
As to those crossovers, if for example a main speaker has good measured response at 40Hz, the crossover to it will be set there, but that isn't necessarily the best way to leave it, just because it can do it. It's probably better to raise that crossover manually to 80Hz, to give the sub more of what it does best and not force the mains to cover those lowest bass frequencies.
