Shag, that LPF is the low pass filter which affects only the channel which carries the low frequency effects which are occasionally present during movies. It has nothing to do with the bass redirected to the sub from speakers with a crossover rather than full range. The LFE channel theoretically extends as high as 120Hz, but in practice little or nothing is there above about 80Hz and most is much lower.
Some suggest that the full 120Hz setting be used, in case something useful might be there in some cases; others suggest setting it at 80Hz to lessen some 120Hz hum or other noise that might be up there. It doesn't appear to be a crucial point either way, but I'd be inclined to set it at 80Hz.
