A sub, used as an ".1" speaker, is independent of the 5 other channels. So, used as the LFE (".1") channel, the crossover frequency is not very critical, since it is not sharing the same content as the rest of the system.
"low pass" probably turns on and off the low-pass filter, which then allows you to set the frequency of which it begins to cut out the higher frequencies. Definitely need to have this switched on when listening to non-5.1 music. I would then switch it to 40Hz, because as you said, that is where the mains are done.
I would always keep it on, anyway, just to spare the speaker from frequencies beyond its capabilities.
"24dB" probably indicates the slope of the crossover (steeper or smoother). Try switching it to see which is better for your room and setup.
"Music" must set some internal parameters for the possible use in a music-only (i.e. non-5.1) setup. This is where the sub interacts with the main speakers, and setting the crossover and slope is important.