What does the LFE switch do? My manual is vague and just says to follow receiver manual (also vague). Only thing it says is that if LFE is on, then the volume control on the sub won't work. I'm using a line level connection. Is LFE good/bad to switch on and what is it?

On mine (JBL PB-10), the LFE switch disables the crossover setting and allows the receiver to set the crossover...

Volume control/level control still works normally...