The LPF refers to the Low Pass Filter of the LFE channel exclusively in multichannel tracks such as Dolby or DTS 5.1/7.1. The LFE channel has a 120Hz ceiling and the LPF should be set at 120Hz to avoid clipping those signals in the LFE channel. Speakers crossed at say, 150Hz, will still have the signals below the crossover setting sent to the subwoofer for output along with the signals within the LFE channel. With an 80Hz setting for the other speakers, the subwoofer will output signals in the LFE Channel up to 120Hz and also the signals below the 80Hz crossover from other speakers. The speaker signals taper off at the crossover point rather than hit a hard wall like those in the LFE channel when they hit the 120Hz ceiling. The subwoofer crossover knob should be set at maximum when an AVP/AVR is handling the crossovers.