ok... taking a deep breath...
nt, cel, and crazy all are right. to a degree.mostly...
so, you should be fine with just the LFE setting, based on nt's post #8 : LFE : The low range signal of the channel set to “Small” speaker size is
added to the LFE signal output from the subwoofer.
what it doesn't say ... in this setting, the re-directed lows do not also play through your other speakers.
LFE+Main : The low range signal of all channels is added to the LFE
signal output from the subwoofer.
what id\t doesn't say... those low notes also play through your main speakers, as well as your subs. You don't want this. It is similar to the "double-bass" setting, and it is ... not really what you want. Maybe. That setting may sound better in your room, but IMO that's an indication of other problems. So it could act like a 'band aid" in certain instances, under certain listening conditions. And if that solves your problem... you're happy with the results.. then stop.. you're done.
But it's not really the best solution... again, IMO
the LFE channel is not only on blu-rays, but also on all dvd's with a dolby or dts X.1 soundtrack. It is the Low Frequency Effects channel. Set the LPF of LFE at 120 . That's different than the crossover chosen between your speakers and your sub(s).
so recapping... setting your system to LFE, (like you have it already) will allow you to replay that channel, via the subwoofer, like it should, as well as setting a crossover of 80 hz re-directts any low frequency content in the other ( not the LFE) channels to the sub. This is what you want.
finally, I wouldn't follow Def-Techs recommendation like Crazy quoted, without some ... qualifiers. As listed above (by Commys...) they list a -3 db point at 57hz . BUT they recommend crossing those same speakers over @ 100hz? Makes no sense at all. Unless they can't reproduce below that...
The best answer is always what sounds best to you, but knowing how the different parts interact, and exactly what each setting does, can not be underestimated.
Greater minds than mine will surely chime in.