Fmods are 2nd order high or low xovers (unpowered active - I know it is an oxymoron, but that is as close as I can explain it) at a fixed frequency. They can be cascaded into 4th order x-overs (using a -3db module and two 2nd order Fmods). They can do the job, but their x-over points are limited, which means that you have less tuning abilty, and you won't get the most out of your system. Higher power Sundown 4 channels have 4th order x-overs. Use those if you can. JPS