crossover frequency is the frequency at which you want the receiver to send data to another speaker or driver, in this case the subwoofer
generally crossovers for subwoofers are set to around 80-100Hz, but if you're using smaller front speakers (i.e. energy micro) you might want to set it a bit higher (i.e. 120-150)
usually this is set on the receiver
does the wharfdale accepts speaker output to the front left right?
if it does, then that's probably how the crossover is set when connecting speakers to the subwoofer directly