After using an active crossover and 4 passive woofers I built based on a design by Roger Modjeski I decided to try active subwoofers and purchased 4 KEF KC62 subs with balanced force drivers. KEF has an interesting take on setting LP and HP filters that you can read here:
Setting the LP and HP at different frequencies worked well with my box speakers, but not so much with my Quad ESL which have a resonance peak at 90 Hz, so setting HP and LP at 100 Hz was ideal. As the KEF article states with a 4th order filter the highest attenuation of the signal will be -24 dB at one octave above for the LP and one octave below for the HP. So for 60 Hz that will be either 120 Hz and 30 Hz.