I am by no means an expert, heck the rest of these Axiom guys/gals helped me along the way
Anyway, when you set up your system, the sub is designed for LFE (Low Frequency Effects), basically the reaaaaalllll lowwwww bass. You know when your watching an action movie with lots of explosions, when your whole house rocks
The crossover is the frequency in Hertz (Hz) where the low sounds are routed to the sub. If you look at the specifications for the speakers, it will give you a range of frequency that the speaker can achieve. For ex, the M22's are rated I believe down to about 50-60 Hz on the low end, and upto 22kHz. So when you set your speakers to small, it will ask you what crossover setting. 80Hz is a good rule of thumb most follow, I believe THX recommend this setting. Also, setting speaker to small, does not have anything to do with the physical size of the speaker, it is actually the ability for it to reproduce adequate low frequencies.
With the crossover set to 80hz, any music/movie sounds that fall below that will be routed to the sub, anything above that will be played by those speakers.
I miss being a newbie, however, still consider myself a big newbie
